Originally Posted by Jiro

Here are federal gun laws (source) -

Federal law regulates these sales through federally-licensed dealers; and the identity of the purchaser and terms of the sale are recorded on Form 4473. Dealers may be inspected once a year, and must be inspected at least once every three years. They must also notify the FBATF anytime multiple handguns are purchased from a single individual within a five-day period.

Gun shops are required to submit Form 4473 to the ATF.

States primarily regulate firearms sales between individuals, but federal law restricts interstate sales: Only individuals with a federal license can sell outside of their own state. They can, however, transfer the gun’s sale through a licensed dealer in the other state. Individuals may lend and borrow firearms provided they’re used in legal activities.