I think it's gut feeling more than anything. I'd be very, very wary of buying tickets through somewhere like Craigslist at all, and if I did go through a site like that, I'd insist on meeting up to exchange cash and tickets, just to be on the safe side. On RAWK I'm happier to pay before receiving the tickets if I can't meet the seller, so long as they've been vouched for and don't have a two-post history. On the occasions that I've done that, twice I think, I've not had a problem.
Caveat emptor.