Cant you get the ATF or some other federal agency involved? If as he claims the rifle was lost or stolen in transit I'm sure the feds would like to know about this and I would think there has to be some law about fraudulently selling a firearm? If as he claims he shipped it, it would have to be by way of an FFL, he would have to have documentation proving it.
I bought a muzzleloader and as it turned out FedEx delivered it to the wrong address although their website simply listed it as having been delivered and signed for, but I was home at the time listed as delivered/signed for and I told the seller this and he promptly contacted FedEx and got a bit of a run around, but after he mentioned it was a muzzleloading firearm and getting the authorities involved in the search, FedEx promptly in less than 48 hours located the people that it was mistakenly delivered to, and had it in my possession.
The people who signed for it knew **** well it could not have been for them after all it was a large package over 4' long by 10" wide and 4" deep. Thankfully it was in still perfect condition.
Sorry for your loss. getting ripped off truly sucks.