I don't know if I should post the scammer steam_id on here but here is what had happened.
He first approach me saying he want to trade with me, since I'm a new in the TF2 trading world, I accept it without considering if he is a scammer or not. He propose a trade for his "festive scattergun" (worth about 16 keys at backpack.tf) with my Poisonous Bubbles of Regret Taunt: Battin' a Thousand and Disco Beat Down Dread Knot (combine worth about 10 keys) plus 1 extra keys to him.
The next day I want to trade the "festive scattergun" on backback.tf , it just said it is a festivized scattergun (worth about 20 ref) rather than a festive scattergun. So I tried to find him on steam and ask for details, but guess what, it just said I and not longer his friend and blocked from messaging him.
I got scam big, real big and hard.