I've bought and sold a lot on ebay as it's the largest market place for parts, new and used. I agree with Olds64 and use paypal as often as possible. Sure they charge a couple percent for a fee, but I understand they will refund something like 75% of what you paid if the guy turns out to be a flake. I also like having the payment transferred electronically rather than a money order sitting in my mailbox until I get home, or me driving to town to purchase and mail a money order.
Feedback isn't always a sure thing as Chumly says. A couple years ago I bought an aftermarket distributor from a dealer. It took a couple months, lots of emails, several phone calls, and once the wrong distributor shipped to me. After I finally got the right one they kept sending me emails asking I post feedback on them. So I did, I said the parts were nice, the price was great, but the service was poor. He then posted negative feedback on me!
When I contacted ebay they said they didn't get involved in disputes over feedback!!! So either I post positive feedback or none at all... even on the 1 out of 100 transactions that go sour.