I'll add my experience also. I had a guy in Anchorage Alaska disappear on me after bad trade. Deal was worth about $300. He got my stuff but I never got his stuff and never heard from him again. I filed a complaint with the
Internet Crime Complaint Center (IC3). About a week later I got a call from an FBI agent in Anchorage telling me I should be getting my stuff soon. About another week later my stuff showed up and the guy re-appeared again on the forums. Amazing what a knock on the door from the Feds will do (yes, the FBI agent went to this guy's house

).
The agent was very nice and followed up to be sure I got my stuff. He also explained how IC3 works. Once the complaint is filed it is dispatched to authorities with the proper jurisdiction - either or all of the following: your local police, scammer's local police, FBI. As someone suggested earlier, it is true. A lot of times it depends on who gets the case as to if it is followed up on. In my case, it went to the Anchorage FBI and a particular agent who decided he needed to know if this guy was small time or if he was part of a larger operation. Lucky for me. My scammer was small time but the local authorities had the resources to investigate it properly.
Anyway - my suggestion to anyone who has been ripped off on the internet is to file a complaint with
IC3. It is a more effective way to log and document multiple complaints against the same person. If enough people file it should warrant an investigation.