You can report them by using our support form:
if you traded with Request a Trade:
if you traded with elite*gold:
This will create a new thread in the correct section. There is no other way to report a scammer or warn other users.
If you want to warn other users, you should also rate him negative (-tbm) [A trade (Request a Trade) or transaction (elite*gold) is required to rate someone negative]
Also you should read this: