If fraud has occurred, you must do more than leave negative feedback. Fraud is a crime and should always be reported to the proper authorities. While eBay may lend assistance in some cases, take it upon yourself to make sure that no one else is defrauded and get the information to the proper authorities as soon as possible. Here are several links that should help you get started:
Check out the Reference Section of this document for more information on Reporting Fraud and Dispute Resolution.