This thread (and the other stickies at the top of S/T) outlines some pretty simple do's and don'ts for trading: http://www.newschoolers.com/ns/forums/readthread/thread_id/459093/
Trading's a little different than making a purchase obviously, so it doesn't have some of the built-in checks that money transfer companies employ (thinking of paypal giving you 90 days to make a claim or whatever it is). I think if you're smart about it though, you can cover the fundamentals and make sure you're not getting fucked over e.g. getting tracking numbers for shipments, email addresses, phone numbers or facebook profiles. You want to set up some sort of a safety net for yourself so that if shit heads south you can hold the other person accountable and not just let them slip away. And also just have a little faith in humanity.