From Milwaukee Journal Sentinel:

A Wauwatosa man is under federal investigation for selling guns without a proper license, buying more than 500 firearms from an outdoors store and then selling them through a website or at gun shows, according to newly unsealed court documents.

According to the search warrant, the suspect obtained a collector of curio and relics license in 2014 from the ATF. The license allows the sale of guns that are at least 50 years old but does not allow the holder to be a general firearms dealer.