I've sold guns 3rd party.
Check their ID to see if their a resident. Use common sense. I'd recommend a written contract and to write the buyer's name and maybe contact info on it. Keep it for a while or not. It's your call. I don't have any of it anymore.
Rule of thumb. Be armed during the transaction. Don't give a loaded gun to someone and don't have ammo readily available where they will be at during the transaction. Think of the Terminator gun store scene