CYA -- Lots of Paperwork
I have always bought and sold guns with just a simple handwritten receipt identifying the gun by make and model (not always the serial number, but usually), the names of the buyer and seller, the price, and a statement that the full price has been paid at the time the gun transfers (so nobody can say that some money is still owed in the deal).
Now that I'm older, wiser, with a legal education, and with slightly deeper pockets, I am a lot more nervous about selling guns to strangers who answer my classified ad, or who are " a co-worker of a friend of a friend" that I don't know myself.
My new policy is that my "Bill of Sale" looks a lot like a form 4473, specifically identifying the several categories of "prohibited person" and the buyer must state that he doesn't fit into any of them. And the form also includes a statement that the buyer is satisfied that he knows how the gun works, how all the safety features can be used, and that he generally knows about safe gun handling. Finally, the buyer is notified that the gun is sold "as is" with no warranty of any kind. (I always offer to meet the prospective buyer at the range and let him shoot it, if he pays for the range time & ammo, if he wants to check the gun's function before buying it).
If I have modified anything safety-related about the gun, I specifically write that into the bill of sale and have the buyer sign that he is aware of it, and that he will further inform anyone who handles this gun about the modification. An example would be a trigger job that significantly lightens and shortens the trigger pull from factory specs. Or removing a "magazine disconnect" feature.