The Postal Inspectors would investigate this as mail fraud. They may get other agencies involved as well since it involves a firearm. I would recommend to give it a shot. Heck, even contacting the PD local to the seller may result in at least a little pressure if not more.

ETA: upon closer look, I do not see FCSS mentioned anywhere in the ad. It can still be assumed with the false mention of the Accustock though. The model differences and resultant values would have to be explained to whomever you call to substantiate why you are reporting the fraud.