There are a variety of government and private agencies that can, in theory, help victims of fraud. "In theory" meaning the reports are not always pursued. There is a very simple reason for this: consumer-to-consumer transactions across state borders have never existed until the recent growth of the internet. The sheer number of transactions limits the agencies' ability to pursue complaints. The "higher up" the agency, the less likely it is to pursue any single complaint.
The best strategy is to start with local agencies, such as the police departments (local to you plus the one local to the other party). The local police are the most likely public officials to pursue a single complaint against a single person. As one might imagine, this varies dramatically from city to city.
The Better Business Bureau may be of some help if the other party is indeed a business. However, the most effective use of the BBB is to check a company BEFORE a transaction.
Each state's Attorney General's office may be of some help, especially if a large number of complaints are filed, or if the dollar figure is high enough.
National agencies are generally limited to pursuing complaints of widespread fraud, but there are some online mechanisms for complaints to be filed.
Below are some links to relevant agencies.