If you're talking about EBay, I ONLY do it one of two ways:
1) I feel reasonably safe ONLY if it's a business, with an address, storefront, in the phone book, etc., with LOTS of feedback. If I can't call long distance information and get a number, call them to check with no mention of the auction, etc., if they're NOT listed as a dealer on manufacturer's dealer list and so forth, PASS. Just PASS.
2) I ONLY buy from an individual IF it's within driving distance where I can go there, meet face to face and see the article in question.
Otherwise, it's a crap game I'm not willing to ante up for.
When I found the last of my 3-Yamaha set, the buyer happened to live in Knoxville, TN, a really pretty 2 hour drive from Nashville. Met for lunch, made a friend, I got the bass, he got the cash, came home happy. But I would have NEVER just sent the money and hoped to see UPS in a week or so. Ain't happenin'!!
If it's not EBay, I do a lot of the same homework as in Number 1 to check out a business before I EVER send the money.
Rami put it in a nutshell: KNOW who you're buying from.
Actually I always try to buy within a 3 or 400 mile radius: GREAT excuse for a little day trip.
J o e y