steam trades currency and where to get them

Same here,I would like to know this too.

I prefer working in keys. Because I can't be bothered to verify my Paypal account, and I usually can't end up doing fair game-for-game trades because I rarely have extra copies of games

You can get them for around 1,8 $ pp at steamtrades ;)

I'm confused. What do you mean by "key value is lower than the store"?

Keys don't 'drop' in games and can only be purchased from the store. That's why they 'retain' their value and are used as currency. At some point of time, people who made profits off trading wanted to convert their keys to $, so they went on to sell their keys for slightly less than the store (so people would buy them). From there, undercutting (wikipedia is your friend) occured.

Key seller from steamtrades here, keys are usually bought from long time traders/players that are cashing out, since they don't want to have to deal with the tons of people who are too lazy or untrustworthy.
Another thing that's too blame for the abundance of cheap keys is the fact that the TF2 Manco store used to have regional prices, which made them even cheaper to get. (this has been changed by now and you can slowly start to notice that people are asking a little bit more for keys nowadays)
Hope this answers your questions OP.

my 70 trades didnt involve those "tf2 keys and refined metal"...

tf2 keys are the same price on ST as on community market, at least if you deduct the fees valve gets from selling there.
sometimes its cheaper on steamtrades because people want to cash out for real money without waiting too long and they make a good price to sell fast all of their keys.

the reason games are way cheaper offered in tf2 keys than their value on steamshop simply is because of regional pricing.
they buy steamgifts cheap on steam --> trade for tf2keys in value little higher than what they paid for the game --> cash out the keys to get back the money --> profit

to acquire some keys for trading look for trusted traders who want to cash out on steamtrades or go to sourceop trading forums

Lowered supply and high demand make keys popular forms of currency.

Supply is kept low by three major factors. First is the fact that they all originate from the same source, the ingame store, which sells them for $2.50 (I am pretty sure) USD. They can be traded, but the point is that somewhere, at some time, someone spent $2.50 on the key, and thus keys are only "creatable" by a portion of the playerbase. Second is that they are consumable items, and so many that are purchased from the ingame store or traded for are immediately consumed and removed from the market. Third is that keys are hoarded by many people as currency for larger trades.

In turn, demand is kept high for related reasons. Not everyone can buy things online (kids, people with no credit or debit cards, people with no access to a place which sells Steam gift cards, etc.) and many who can simply don't want to spend real money on a free to play game. This gives people who buy keys the upper hand in trades, and a potential to make profit. Of course the same potential is available to people who trade for keys ingame, provided they got a good deal on their keys in the first place (which is difficult, as most people who buy keys either intend to use them or to trade them for profit). Of course keys are not just shiny things to keep in your inventory. They open an item called a crate containing a random item from a specified list, including a 1% chance for an "unusual" quality item (worth 10 keys minimum, over 1000 maximum). This sort of gamble makes a nice place for some to sink their disposable currency (whether IRL or ingame).

With all of that said, keys are one of the easiest virtual items to sell for Paypal (which most consider to be as good as, or sometimes better than, cold hard cash), and they probably will be as long as TF2 continues to thrive (and even then, DotA 2 has a very similar key system, with many traders already taking DotA keys 1:1 with TF2 keys). Of course they can't be sold for the same price as Valve sells them. When not buying from Valve, there is always the potential for scamming, as well as competition from other traders. This drives resale prices down to where they currently are.

There is also the Steam Marketplace, but those funds don't spend quite as well as Paypal and regional game pricing often makes it better to buy games from a game reseller than to buy them with your own wallet funds (provided you do the research on region locks and whatnot). Marketplace prices fluctuate but tend to be lower than the $2.50 of the ingame store yet higher than Paypal values (again, the lower price attracts people away from the ingame store, while the fact that Steam wallet funds can only be used in Steam means key sellers will likely charge more for that inconvenience).

As for where people get keys so cheap, first of all salesmanship is an art form to some. Buying low, selling high, convincing people they need what you have more than they need what they have, some people do it for fun. Some people do it all day. Where the real money is, though, is taking advantage of regional game pricing for profit. Certain countries, especially Russia, have notoriously low game prices (supposedly due to lax and/or nonexistent anti-piracy laws, which force companies to sell cheaper to better motivate people to buy rather than pirate). Some games are region locked because of this, but many are not. Basically, a game reseller (one of whom I spy in this thread! :P) will buy the game and sell it for keys or for Paypal at a price that is somewhere between their region's price and yours (again, usually charging just a bit more for the effort of converting the keys to Paypal themselves).

I hope that answers everything.