All it takes if you end up the victim of a fraud is going to the police and the district attorney and filing charges. If it involves altered serial numbers on the receiver, ATF may also like to be involved. I also recommend filing a civil suit, even if it is in small claims court with jurisdiction.
As long as people think they can get away with these kinds of things, they will try. Bring criminal charges when you are stung by a crook and they sure won't in the future, as much because felons cannot own firearms as anything else.
