Does he have a permit to buy and sell weapons within the United States? He is not an American citizen, and lives in Canada. At the very least he is breaking International trade laws
No permit is necessary to buy and sell weapons if it was for his own collection. There is no evidence yet disclosed that he owned the gun shop. No article states that the guns were in international trade. They went from a shop in the US to a storage unit in the US.
A Canadian citizen who is in the US legally can buy firearms provided certain paperwork requirements are met.