First and trade scam.

Asking advice on how to scam someone?

Check steamtrade feedback and steamrep reputation.

Can you give us more details ?

Trade with people who have good reputation. Make sure you're trading with the real trader and not with an impersonator. You can do that by going to someones SG profile and click the button that leads to their steam page. Then compare the steam page of that person with the steam page of the person you're about to trade with to make sure they're the same person.

Yes, was a fake profile

the "enhanced steam" add on, add some usefull link on steam profile to check some feedback

Having some extreamly boosted prices (example: Darkness 2 is not worth 10 keys, rather 1-2) will only attract scammers.
Nobody with decent reputation and knowledge about trading will pay you 10 keys. Only a scammer can "offer" that amount and run away with your code.
I am not saying that if you sell Darkness 2 for 2 keys it will result in a clear trade but surely the % of fake offers will sink.

if you're new to trading, barter.vg is a great tool in my opinion
for example: https://barter.vg/u/3bb/
(Ignore the "fullfillment rate" stat, it's absolutely irrelevant right now)

If you want to scam people follow these steps:

• Find a person who's willing to trade something low value for your value key
• Get him/her to go first
• Then drop your Shadow of Mordor key and before he realizes what he got instead of the bundle game he wanted, close chat and block.
• ???
  -Profit! Stalk the profile and enjoy that he's forced to play the game instead of just idling the bundle game for cardz, robbing him/her off of the precious free time :3

Posted this for someone else before, might be useful to have a look:

Checking:
1).Always double check the trade feedback (e.g http://www.steamgifts.com/user/maildave/feedback) + the steam profile that it links to, and the person you are talking to on steam (ask them to post on ur thread to double check too). [Normally +100 rep and above rarely scams, +50/-0 rep and above are decent and you can probably trust still, +25/-0 rep and above are still okay, anything below +25/-0 try to do more checks and always check anyone with -rep and take extra care]

2).Check steamrep.com (not so accurate sometimes, as only confirmed scammers, with who had done a few scams are shown there), this site shows confirmed scammers who you should never ever trade with (if he is shown as a scammer on the site)

3).Check if the person is in any rep boost groups (most of the time those who are in rep boost groups on steam (e.g of rep boost groups https://www.google.com.sg/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=%2Brep%20group) scams when they have the chance)

4).Never just go 1st based on steam profile reps/ inventory alone (as steam profile reps can be easily manipulated, as mentioned in point3).

Trading:
1).Normally most trades done here are game keys for cards, refs, other game keys, csgo/tf2 keys, so most of the time 1 person needs to go 1st. As a standard rule of thumb, normally the lower rep on steamgifts always goes 1st. (Unless both players are around the same rep) (Of course you could ask another person with higher rep to go 1st, and some times they would if the item is of low value)

2).Try to build up some rep by trading lower value stuff (e.g bundled game for cards) or safer trades like gifts for keys. (At the start it might be difficult to find trades without going 1st due to low rep, so I would suggest you slowly build up rep this way 1st, before selling higher value items/games to avoid getting scammed)

3).Sometimes trades might take a few days to a few weeks (with constant bumping a few times a day everytime u can, you can bump every hour) before people will post on your thread (if your trade is not super value or quick sell, i.e very cheap compared to market), [There are many people who only look through the 1st page of the trading page, so they only see ur trade when you bump it]
So it might be good to search around other people's trades to see if there are any trades you might want to do with others if you want a higher chance to trade away your game.