Used-firearms sales aren't regulated at the federal level if neither party is a federally-licensed firearms dealer, according to the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF). The seller of the firearm should still produce a bill of sale, including a serial number and weapon description to document the transaction.
When a firearm transfer occurs between two private parties, the transfer must happen between two current residents of the same state. The seller must have no reason to believe that the buyer isn't allowed to possess firearms. Felons, people that have been deemed by the courts to be mentally defective, anyone that has been convicted of domestic violence and people living in the country illegally are barred from gun ownership by federal law.
State laws can require additional documentation or background checks. The seller must examine a government-issued identification card from the buyer such as a driver's license or state identification. When drawing up a bill of sale, the name, address and identification number should be included as well as the serial number and weapon make and model. This protects the seller should the weapon be used in a crime and protects the seller should the buyer claim the weapon was stolen. The bill of sale should be photocopied and signed in duplicate by both the buyer and seller, so both parties have a copy with an original signature.