I would like to point out one area that appears to have been missed so far. Check with the nearest bankruptcy court to see if he has filed bankruptcy. If he has, then you would be considered a creditor and need to file a claim with the bankruptcy court. I am not a lawyer and I do not know if you are considered a secured or unsecured creditor. There is a difference in your chances of recovering your property.
Also, and I hope this is not true, make sure the actual owner of the shop has not died. If he has, your property is tied up in the probate courts also, very similar to a bankruptcy. A relative may be the one selling his guns off.
My first thought other than that is to agree that you should go knock on his door and ask him for your firearms back.