Encountering non-paying bidders can be very frustrating.  If you have an auction close and then the winning bidder immediately backs out, or even if they pay and then immediately demand a refund -- that bidder may be attempting to defraud you of your listing fees and may have the intention of leaving you an undeserved negative feedback.  These people who do these things are thieves.

If you encounter a situation like this, you should immediately leave a negative feedback for that bidder, because ebay now requires everybody who registers (even only to bid) to use a valid credit card to register.  What this means is that if you take a firm stance and don't worry about a retaliatory negative, others will take the stance, too, and these people will be weeded out.  Ebay also tracks IP addresses, so if somebody uses his malicious account to defraud you of your listing fees, and you are sure to leave him the appropriate negative and report the incident to safeharbor, his other account(s) will most likely be suspended as well.

Also, be sure to file the appropriate non-paying bidder forms in the ebay system, and as a seller, you can block bidders who have x number of non-paying bidder warnings against them.  If you do it with your non-paying bidder, then he won't be able to maliciously damage my auction.  If I do it, he won't be able to harm yours.  If I leave him a negative, you can cancel his bid and add him to your list of blocked bidders.

People need to take a stand against these bidders with malicious intent.  If they are weeded out, ebay will be a much better trading environment.

There are methods to prevent these people from doing this, and with a good foundation, and a little time, these malicious accounts will be permanently suspended, and the owners of the accounts won't be able to register a new malicious account under a fake name because they need to use their own valid credit card to set up a new account.  Please inform your ebay trading partners about this guide.  Thank you.