The problem with this, is that this can very quickly turn into a witch hunt and new members whose purpose is to do legitimate trades, are going to have a hard time if everyone is going to warn about them in their trade threads. At some point you're going to accuse a legit guy... or gal.
Instead of calling out possible scammers, a long sticky guide on how chargebacks work, things to look for when it comes to suspicion about scammers, common sense, other tips and tricks, etc, would be a good idea. Lucky Ace (2dgreengiant) has already made a
similar (New trade section info and reading material as of 12/08/2011) thread about this last year, but's a bit hidden (WoW trading section) and probably slightly outdated.
1) Don't report members you
think are scammers, we're not going to do anything without proof.
2) BUT please do report suspicious threads where all the feedback are 1rep, just registered users.
3) Don't call out users in their threads with little to no proof.
Use your common sense, stay away from "1rep, just registered" members if you're paranoid/afraid of getting scammed. Check their rep and judge yourself if you think they seem legitimate.