His M.O. is almost always the same, with slight variations. He gathers beta keys and offers them as full operating final game keys, knowing they are not. He also offers stolen keys that get invalidated or revoked afterwards.
If you confront him, he will use one of his canned English sentences "giveaway code sorry" or "i not problem" (meaning it's not his problem) or "company credit card cancel" (yeah, go figure).
All known profiles to date, as well as history of his scams to me and to other traders can be found here.
He's gone so far as to attempt to impersonate me and other traders to disguise his actions.
+rep, didn't have one of the 12 we traded but wasn't a big deal, trade was fair and even 1:1, would trade with again
Day after the trade the assured full key was revoked!
they did offer another game.
but they seem to specialize in given away games and it was not worth of the original trade.
Gave me a Beta key for my game. I specifically asked him if there is danger that the key gets revoked and he assured me that it's a permanent key. Few days later the key gets revoked. I asked him what he wanted to do to make the deal worthwhile for me and he offered a selection of previously free keys. After telling him that I'm not interested in that offer and that I wouldn't be unreasonable but he had to give me a better offer he told me that this was his first and final offer. I made screenshots of all the conversations and the revoke screen. His Steam profile is now private.
Initial conversation where assured me the key is good and won't be revoked:
Conversation after the game gets revoked: