Have a two-copy Bill of Sale with names, addresses, date, description of item sold, amount$.

Have buyer bring a copy of his paperwork for you, such as photo ID with current address, age, like a driver’s license, and state permit/license if available to establish identity.

If he comes by car face-to-face, photo the car and plate. Do it in the parking lot of the local police station. If payment is in cash, photo it next to the gun. Have good pictures of the gun with the serial number, make & model showing.

Keep a (digital) file of the transaction and you can prove you made a legal transaction, according to the best of your knowledge, if you ever get questioned about it.