My Steam account has been hacked twice (I used to use an old virus-infested computer owned by my brother). I contacted Steam support with a ticket (twice) and got the account back. You can only get TF2 items back after one hack. I got mine back, but the 2nd time I couldn't. I permanently lost extremely valuable stuff (multiple unusuals & buds & stranges & stuff). Lots of games, too.
Valve asked me to supply the last 4 numbers of the credit card I used to purchase stuff on the account, maybe that would help you get the account back. Try your best to get into contact with the person you bought the account from, as he might be the one who stole your account.
Good luck getting your account back!