This article is from the eBay FAQ, by William B. Shaw firstname.lastname@example.org with numerous contributions by others.
Your options depend greatly on a number of factors such as how you paid, the
dollar amount, and where you and the seller are located. First things
first - contact the other party and make sure there hasn't been some sort of
misunderstanding. If you aren't receiving results, contact eBay, your credit
card company or payment service, and any appropriate law enforcement
officials. If you mailed your payment or the package was expected via US
Mail, see http://www.usps.com/postalinspectors/fraud/ to determine if mail
fraud statutes might apply and you can file a complaint with the United
States Postal Service.
Generally, it is very difficult to get cooperation from law enforcement
unless the dollar amount is in the thousands range. Many times your recourse
is through the civil court system which can be expensive and can take a very
long time. The Federal Trade Commission has a very comprehensive guide here.
A fairly extensive guide to both buyer and seller options when an eBay
transaction goes wrong is listed at