It's rooted in federalism. The federal government generally doesn't have the power to regulate transactions between individuals within a state. When it has assu

It's rooted in federalism. The federal government generally doesn't have the power to regulate transactions between individuals within a state. When it has assumed such powers, it has required some legal gymnastics. A gun dealer with an FFL has a license to do interstate commerce in firearms and is subject to federal regulations on the subject. A private sale is governed by state law.