To save there being multiple stickies at the top of the Scam Accusations forum, here's one big one.
Before you make the transactionRead
Tomatocage's guide on spotting scammers.
While making the transactionUse an escrow (a middleman who makes sure that each party fulfilles their promise).
Popular escrows:
John K's EscrowAfter you've been scammed or the transaction completedMake sure to
leave a trust rating, and make sure to make it
negative if the person scammed you.
Also, if you've been scammed, make a post right here, in the Scam Accusations forum!
If you have any problems, errors or information, please make a post below and I'll be sure to take a look!