I believe that, unless this guy is a licensed manufacturer of firearms, he is debarred from assembling these guns for sale, therefore....HE AIN'T DOIN' IT FOR THE MONEY!!!!
Technically, you can assemble firearms from parts for your own personal use. You can even completely manufacture your own firearms for personal use. You can even then sell those firearms that you built for personal use, provided you do so legally and inscribe a “serial” number on there. However, like everything with the government, if you’re doing such to any capacity without a license, you’re darned right they will make you a criminal just because.