Firearms may be lawfully made by persons who do not hold a manufacturer’s license under the GCA provided they are not for sale or distribution and the maker is not prohibited from receiving or possessing firearms. However, a person is prohibited from assembling a non-sporting semiautomatic rifle or shotgun from 10 or more imported parts, as set forth in regulations in 27 CFR 478.39. In addition, the making of an NFA firearm requires a tax payment and advance approval by ATF. An application to make a machine gun will not be approved unless documentation is submitted showing that the firearm is being made for the official use of a Federal, State or local government agency (18 U.S.C. § 922(o),(r); 26 U.S.C. § 5822; 27 CFR §§ 478.39, 479.62, and 479.105).
Additionally, although markings are not required on firearms manufactured for personal use (excluding NFA firearms), owners are recommended to conspicuously place or engrave a serial number and/ or other marks of identification to aid in investigation or recovery by State or local law enforcement officials in the event of a theft or loss of the privately owned firearm.
So there may be a grey area regarding intent when making the gun. If the gun was made for personal use and not for sale, but then later sold...
dangerous waters there. Personally I would never sell a milled out and completed 80% lower. As soon as you sell a homemade gun the government can show intent to sell...cause you just sold it