I don't think Steam can ban him for doing that as Steam doesn't really support trading but anyways, I hope he can pay for what he did.
www.Steamrep.com and it looks like you were his first victim. No signs he's scammed other people
Sorry to hear this! I always ask for their steamtrades profile first to check their rep before I go first in any trades, I don't think steam will be able to do anything because technically it wasn't a scam to them and you just gifted him portal 2, best case scenario is that he will get trade banned although that is unlikely, just make sure you took a screenshot of the conversation!
You ALWAYS have to check his Steam Trades profile (and check that it leads you to HIS Steam profile) before making a trade. And if you see he has 0 rep, you never go first. That should be your number one rule. I personally don't care if the person I'm trading with has +20 rep, I won't go first unless he has more than me.
And please remember always checking and making sure he is an honest trader before sending something first.
http://steamcommunity.com/profiles/76561198089240393
I gave him Portal 2 gift, he said he will give me a cd key of Call of Juarez Gunslinger. I went first and then he dissapeared, he scammed me. i sent a ticket to steam support, I hope this idiot will get his Vac ban and I will get my Portal 2 back.