It must be the 24th of the month.... this topic is about as regular as my wife's...
I've bought from every place mentioned previously in this thread, except ebid or craigslist. Never had a significant problem. Never met a seller either... only dealt through email/mail/paypal. My secret (that I don't mind sharing) is not being an extreme bargain-hunter and not being in a hurry to get something. It seems like the most frequent deals-gone-bad are those in which buyers take excessive risk. Patience is the key -- there only rarely is a truely one-of-a-kind item.
As far as selling, I'm not much of a seller and feel bad for the "small-timer" who gets faced with high fees on eBay. It seems to me, though, that they offer a lot of service for the price, but it does appear to have become a place where those operating on a slim profit margin can't turn a profit.