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You see a Louis Vuitton purse on eBay for a price too good to resist. So you win the auction, then you wait and you wait, but the purse never arrives.

Nondelivery of a product is the most common auction fraud. In Kansas last year, police investigated a woman who was selling designer purses on eBay she didn't actually have in stock. After collecting the money from an auction, she'd go online and try to find a knockoff purse selling for less than the winning bid, then send it on to the buyer. Much of time, though, she didn't bother, and the bidder got nothing at all.

1. Check the feedback.

eBay's seller feedback page can help you tell the reliable resellers from the rip-off artists. Click to enlarge

Never bid without checking the seller's Feedback Score. You'll find it in the Seller Information box in each auction. Once there, click "Read feedback comments" to see what other customers had to say about the seller. If someone is operating a get-rich-quick scheme--or is just lazy and disorganized--these comments will quickly reveal the seller's weakness. Look for a seller who's been in business for at least three months and has a positive feedback percentage of at least 96 percent. Scammers will find it hard to maintain a positive rating for that long.

2. Dig deeper. Click the seller's name on the auction page to get to the Member Profile, then click "Learn more about me." (Not all sellers provide this info, but if it's there, it will show up right below the "Add to favorite sellers" link.) Is there anything more than an e-mail address? If not, be wary. If there's a phone number, call it and ask a question about the product. Or simply click Contact Seller and send a message. According to the Internet Crime Complaint Center, the buyer knew nothing more about the seller than his or her e-mail address in most cases of nondelivery.

3. Buy from those you trust. If you've not bought from a seller before, see if that person is a member of PayPal Buyer Protection plan, which provides buyers $1,000 of protection against nondelivery and a host of other nightmares. Only sellers who have met PayPal's requirements--for example, a feedback score of at least 50, with at least 98 percent of your feedback deemed positive--can offer this plan.

4. Check your insurance. If you bought from someone not covered by the PayPal plan, eBay will refund up to $200 of the cost (minus a $25 processing fee). However, if you paid by credit card, check with your bank; some cards offer 100 percent guarantees against online fraud.