Fair point, but without a tax stamp in hand OP cannot show that the transfer to the new dealer was unlawful.
The legal question here is who assumes the liability, OP or the new owners. Without having the a copy of the pervious owners financials to audit the money trail (which honestly may not exist anymore) it is a hard case to prove.
OP has a not-approved Form 4 and a receipt showing payment of funds, the new owners also have a receipt showing payment of funds and an approved Form 3. That means a civil case against a most likely now defunct LLC for the amount paid, so without evidence of the original owner acting in bad faith OP has no way of going after the original owner himself.
I hope OP gets made whole but I don't see how the new owners are obligated to hand over inventory they paid for because of a mistake of the previous owner. Enlisting the help of the new owners is the cheapest way OP is going to find out what options he has available. OP can sue the new owners, but the cost to have the lawyers do all the legwork will most likely exceed the cost of the items, and if the courts find in favor of the new owners his expenses just doubled.
Arguing over what should have happened isn't productive or helpful, we know what did happen and OP wants to know what, if anything, he can do about it. Seems the only way OP is going to get relief from the new owners is if they acquired the operation whole and the debt owed would have been assumed with it. Those details should be easy for OP to get by asking nicely
Whole lotta undefined variables, that is only given in this shit show.
New FFL / SOT saying they can't help because cans are now their cans or saying can't help because they don't have a form 3 saying it can transfer from them?
"...we know what did happen.."
We don't know shit about what happened. We have a partial, at best, one side of story and NONE of the actual paperwork on assets moved / sold as part of the business transfer.
Huge assumption that cans were transferred on form 3 and now are part of new FFL / SOTs inventory unencumbered. Unless there is a clear paper trail to that effect, I think new FFL / SOT has a problem. Especially if there isn't a form3 transfer to them, which is probably why they don't want to touch this.
ATF - NFA can help by clearly defining WHO is currently the FFL / SOT of record on last form 3.
Been there, done this in a very similar situation before adding SOT to my FFL. Took a good 3 months to get resolved, lotta calls to ATF-NFA, lotta conversations with the new FFL / SOT. I had paperwork, showed the paper trail and issue got resolved. Frustrating as heck and time consuming.
OP needs to get information from ATF-NFA on where can is supposed to be, what FFL / SOT is supposed to be in possession of the cans.
OP needs to take information to new FFL / SOT showing he has receipt for cans. IF new FFL / SOT doesn't want to help, find new FFL / SOT and have cans transferred via form 3. IF new FFL / SOT refuses to transfer on a form 3, then that is a different problem.