It has come to our attention here at TF2S that a well rehearsed scam going around whereby multiple scammers prey on an unsuspecting player to rip him/her off. The game is called "Trust" ironically, and the scam involves a player claiming they are quitting TF2 and want to give away valuable items but only if they play the "Trust" game. This involves other people in on the scam who supposedly also trust the "quitter" and give him items which the quitter then quickly trades back. This goes around until the victim actually gives something valuable to the quitter and then the scammers all disappear.
Simon1123 on SPUF describes and documents the scam here. Obviously with a game like TF2 where items translate to real money, use your best judgement.