You'll want to take this to the scam report section.

Also you'll want to provide some details as to what happened, or else you'll be ignored.

where is the scam report section?

Never tell your password to anyone.
spitfire418: hello
Mathkiller: Hello
spitfire418: I'm interested in your Walking Dead
Mathkiller: okay
spitfire418: I have a Payday 2 key for it?
Mathkiller: okay
Mathkiller: I've got to see if it works first...
spitfire418: but you dont have any rep?
Mathkiller: I don't usually trade
Mathkiller: report me
Mathkiller: if I
Mathkiller: don't give you the game
Mathkiller: I'll get banned
spitfire418: but I'll still be out of a PayDay 2 key...
Mathkiller: and I don't want to lose
Mathkiller: my games
Mathkiller: Dude
Mathkiller: Just trust me
spitfire418: I can't if you have no rep
Mathkiller: ...
spitfire418: rep = trust
Mathkiller: Ugh :(
spitfire418: here's my steamtrades profile
spitfire418: http://www.steamtrades.com/user/spitfire418
Mathkiller: fine
Mathkiller: give me the key
Mathkiller: in the trade
Mathkiller: I'll give you walking dead
spitfire418: that's still me going first lol
spitfire418 has accepted your request to trade.
Mathkiller: write the key now
Mathkiller: It's hard to trust
Mathkiller: a stranger :/
spitfire418: okay I'll write it, and we press enter at same time?
Mathkiller: Send the key now
spitfire418: or that happens lol
spitfire418: okay
spitfire418: let me pull it up on notepad
spitfire418: 3JUU-KJ9S-J43H-8SP8-VV4K
spitfire418 is now Offline.
spitfire418 is currently offline, they will receive your message the next time they log in.

https://steamhost.cn/steamcommunity_com/groups/tf2outpost/discussions/3/

You can find all the subforums to the side of the webpage.

Also you want to read the sticky posted there for format and required information.
On top of that, a screenshot or some other proof may be asked, a copy and pasted chat isn't enough in most cases.

ty