How to be sure if I'm lowballed or I'm lowballing someone?

Moreover, there is a numerous amount of bundles which contain many games and that is confusing me even more on how to properly tell what's the value of each game. For example, a bundle is containing 5 games and it costs 10 EU/USD. Does that mean that each game's value equals to 2 EU/USD? I'm skeptical...

No, it depends on the demand for every particular game. Take the latest Humble Choice as an example. Metro Exodus is going to be much more sough after and worth more than Family Man, even though they are in the same bundle. Consider also that games might have been bundled multiple times in the past which decreases their value further.

In my experience, going by prices on grey markets is good enough, but traders might still not accept an equal value trade, for example if you offer them 5 bundled games for one unbundled game, or if they don't really like the games you're offering (or they want to make a profit on the trade at your expense, I've seen multiple big traders prey on newcomers who don't understand trading very well).

I recommend you use barter.vg for trading, it makes it much easier since you can set clear wishlists, what you have for trade, and get matched with other traders.

Value the games for yourself. If you think it's a good deal, it's a good deal. If you have no idea, then a bad deal most likely. Because at the end of the day, you are the one who will play the games ;)

Personally, I use Steam prices as a reference and a game that I like would worth maybe double the value while a game I don't like would worth half of its value. It is simple and works for me so far.

In general, the price always depends on how much people are willing to spend.
To get a general idea, you can check first on some gray market sites on allkeyshop (like g2a like you said, or kinguin, or stuff) to see what's the lowest price, then you can see at how much people on site dedicated to trading (like steamtrades) are willing to sell and buy.
If the middle price between selling and buying corresponds to gray market sites then that's the price, if it's lower on the trading sites then it's probably a game that you could get for free so the price is even lower than gray market sites.

As for the prices of games that are in a bundle, depends like I said with how much people are willing to spend.
One of the most memorables bundles I can remember it's the Humble monthly/choice that had spyro and crash. Basically people were buying to sell those games nearly the same price of the bundle, and people were willing to buy them because well they were new and popular games, so it was something like 40% crash, 40% spyro, and 20% the other games. The price was pretty high for them since they were never bundled, and probably will never be again.

There are some exception, like old removed games that start to pop up again, steam gifts, and stuff like that.

Use barter.vg as mentioned not only to maintain your tradeables, but also to see how often a game is up for trading and how often it is wishlisted. It's also the best source to check how often and where it was bundled.
For (historical) prices check IsThereAnyDeal for official reseller, AllKeyShop for keysellers (no history though) or gg.deals for both combined.

Thank you all of you for your inputs and advices! I recently stumbled upon barter.vg but found it a bit complicated for my taste and I thought there might be an easier way. It was hard for me to really understand how it's working and what to look for. If that's the best option I will definitely give it a second chance! I'm more concerned to lowball someone than someone lowballing me becasue as Peg4sus said I'm only valuing games from my point of view - if it catch my interest I will play it but seems that's not the case for the most of the traders and I'm ok with that.

I agree with Peg4sus in that it depends on how you value games personally. I never value based off the grey market, or have/wants or any of that. IsThereAnyDeal, imho, is the best source for being able to determine a game's true value for me. Whatever is the lowest price or which bundle it ended up in I would like to believe the devs had some say in, and I prefer to respect their lowest chosen value over randoms in the grey market.

In terms of learning how other users may value games, yes, Barter is a great option as mentioned. Just remember user values vary WILDLY, based on Have/Wants, Rarity, Steam prices, historical lows, users scores, grey market sites, waifu quantity, pancakes, and the list goes on and on.

Don't worry so much about offending people with your offers and instead focus on finding the people that value games similarly to your personal preferences. It will take a while, you will meet a lot of rejection and reject a lot of people, but you will find some great trades in the end.

Lastly, don't neglect Steamtrades just because it's not as user-friendly. Some of my best trades were done on that site, and met some awesome people there as well. Use BOTH barter and steamtrades for the best possible experience.

There are numerous ways to do it. For me personally, I use a mix of ITAD lowest, bundle count, MSRP. Active bundle games are worth less than ongoing bundle games. I most evaluate if I think a game is worth a lot or a little. If something is popular and multiple people wanna trade for it, Im probably gonna hold out until I get something good from a wishlist. If it is a smaller game I'd never play that was part of a bundle with higher priced games, I'll trade it much more easily for anything I'd consider playing. At the end of the day, it's all up to what you think is good or not and you really have to define it.

For bundled games the grey market price is a good estimate for the value as in those cases the past bundles are the main sources for the grey market keys. You can easily check that on gg.deals for example. Another good thing to check is the number of keys available vs. how many people want them on barter.vg
Whether the game was ever given away for free can have an influence, too. Even if the price has rebounded some people might still be averse to trade anything else but other freebies for such a game.

Trading can be hard. Quite a lot of people are reluctant to accept equal value trades. Most people on trading sites have insane backlogs so can be picky even if you offer them whislisted games for their spare keys. There are ofc traders who only trade for a profit and collectors who buy lots of bundles and want to get unbundled games for their spare bundle keys. So it's not that easy to find a willing trade partner. I use barter.vg because there you don't have to interact with people until the offer is accepted. It largely cuts out the bs (like haggling or snide remarks). If people don't like your offer they can just decline with a click and you both move on.

To increase my chances I usually make offers to people with a genuine wishlist first. If somebody has thousands of games on their list they are most likely a trader and the WL is there to generate matches, and does not represent a real want. I also don't mind to 'overpay' a bit. People like to win so it increases the chance that your offer gets accepted. It doesn't really matter if I give a $3 value key for a $2 value key. I wouldn't be selling it on G2A anyhow. My other option would be to activate it and get a few cents from the trading cards. Compared to that getting a wishlisted game of slightly lower value is a huge win.

Welcome to the wonderful world of trading. Your question is flawed because you make reference to "Am I lowballing someone [in a trade]?" You are using a colloquial term. What does lowballing really mean? Your question is "Am I providing an unfairly low bid in a trade scenario?" This same question comes up between 8-year-olds trading baseball cards--there is no clear answer.

The basic answer to your question is: You SHOULD be lowballing if you are trading for profit. What is the alternative: to be unfair to your interests?

If the other person does not like your bid, your response is: "Ok, I can see this is not for you--good luck, I hope that you will find a better trade that will suit you elsewhere. Hopefully, I will find someone who is more desperate than you." In trading, you are not responsible for the emotional frustrations of the other person. "I wasted my time even talking to you." "Well, if we did not quickly come to an agreement, it seems that every interaction is a waste of time until 2 people quickly come to an agreement of middle ground trade value." Trading is inherently frustrating because value is NOT clear.

But who is really trying to make a load of profit from trading bundled games, right? It sounds like you just want to present a fair middle-ground offer. Sometimes you will be undercharging, sometimes you will be overcharging--you want to be close to a zero profit trade. You use the same responses as above when someone is upset. Often, the other party is trying to emotionally PRESSURE you to accept h/her version of values and trade the way s/he wants. So, if you are accused of lowballing--it is a blatant tool of manipulation, end communication immediately and move on. If the other person were truly interested in trading, s/he would present h/her bids, and move on dispassionately as well.

.1. Retail prices on Steam are junk. Almost every game will go on sale for 80% off eventually. And this game was in a bundle.
.2. Yes, basic math works. Anybody could have bought your bundle 5keys for US$10. So, $2/each. But the other trader did not buy the bundle. They either missed it, or more likely, they only wanted 1 game for $2. You sell h/her the desired key for $2 worth of trade, and you are still stuck with the 4 other keys that you do not want (and nobody else wants)? $10/5 does not really work. Often, that 1 key is worth $4 trade.

Narga95 response above is good. Grey markets give the best approximation of value if there is a true bid and offer price.

I use steam market prices to compare bundled games with bundled games. I never accept an offer of less than 15% of the current store price; even for bundled games because I can always use them here.

Whew.

Unless you're young and unemployed, I can't see how trading would ever be worth the time or effort.

It all depends on the trader. Probably 99% of traders are greedy/lowball traders.

I tend to trade based on this list, in order of inportance for me:

1. traders REP
2. removed vs available
3. shovelware vs actual games
4. bundled vs non-bundled
5. How long since it was last bundled vs Historic low for non-bundled
6. reviews.
7. drops cards (usually only for bundle for bundle trades, some asses only wanted my cheap games for the cards to make money)

I wish steamtrades had a feature to track/tag/group users, sort of like steamgifts blacklist/whitelist, so I can mark greedy sob traders.

The constant "My shovelware shit for your........" offers.

edit: ps. I'm pulling that 99% out of the air, no real way to calculate that, it just feels like that at this point in time.

Value is a subjective thing, just like asking if you think someone is attractive. While you can try to index different sources and give them weights to arrive at a number, you'll still encounter people who won't agree because it depends on the person.

Try not to get discouraged. Do your best by checking grey market prices, checking supply and demand (barter.vg), and any historical low prices (isthereanydeal.com). If after that, people give you a hard time, just ignore the comments if they aren't productive. Some people tie their personal self-worth to an activity so then they lash out over something as trivial as a game trade, it should tell you enough.

At the end of the day, it's 2 people in a transaction trying to get the most value they can. Anyone that poses otherwise should ask themselves what they do when it comes time for them to pay for their own insurance, banking fees, mortgage rates, etc. where they will usually get the best deal they can, regardless of other factors. At the end of the day, you can have all the data and opinions in the world, all that matters is whether someone is willing to sell at a price or buy at a price.

I use a combination of methods. Games that have been bundled multiple times usually have lower trade value, but not necessarily always. Check current price on G2A and Kinguin and use that as the highest value for a game. Also check how recently a game has been bundled and which tier it was in each time it was bundled. Rarely will a game's trade value be more than the cost for that tier of the bundle. (E.g., 3 games in a $3 bundle means not one of those games will be worth more than $3). Dividing the cost of a tier by the number of games in the tier sets a good starting price for games that have little demand, but it does not mean that is a good price. (E.g., those 3 games in the $3 bundle may or may not be worth $1, but they also could be worth $0.30, $0.50, and $2.50 based on demand.)

The one thing that I will never understand: users will hold onto a game for years and then expect the game's value increases because they've had it so long. This is simply not true. The longer you hold a game, the lower the demand for that game becomes. Lower demand means value drops. Good luck trading that 5 year old game key (even if it was never bundled again) when no one is asking you for it in trade. (Most recently I saw this with Witch It. This game was bundled only once in Tier 2 of a 3-tier bundle: 9 games for $8.46 at that tier. People are asking 4 TF2 keys for this game when it has been on sale for as low as $6.) It's silly to expect people to pay more for a game than they can pay for it buying it retail.

I'm sure people will disagree with me, but this is my method. My goal is simply to trade away as many of my unused keys as possible. I'm not concerned with making a profit. I'm happy to break even or even trade down if I get a game I want to play.