2005 Maine Code - §359 — Receiving stolen property
1. A person is guilty of theft if:
A. The person receives, retains or disposes of the property of another knowing that it has been stolen, or believing that it has probably been stolen, with the intent to deprive the owner of the property. Violation of this paragraph is a Class E crime; or [2001, c. 383, §46 (new); §156 (aff).]
B. The person violates paragraph A and:
(2) The property stolen is a firearm or an explosive device. Violation of this subparagraph is a Class B crime;
Unless it can be PROVEN that the FFL had, or should have had, knowledge that the weapon was stolen, they can't be charged. The best bet for the OP is to contact the Maine Attorney General's Office, specifically the Consumer Protection Division, and determine exactly what recourse he may or may not have under Maine law. This leaves out the BATF and eliminates the need to hire a private attorney.