Its not illegal to make a profit selling guns. It is illegal to make your living selling guns without a FFL. I don't think he's gonna buy a new house with the proceeds.
Seems to me if he's doing it for a profit it could be seen as operating without an FFL. The ATF doesn't seem to place a number on what's "too many." I have no dog in this fight but it is good to keep in mind.
https://www.atf.gov/file/100871/download