I would hope that you have enough brains to write up a bill of sale and put both signatures on it when you make a private sale or purchase.
That protects both parties.I agree. That establishes a mutually agreed upon transfer between parties and acknowledgment that the seller was paid for the merchandise and the buyer received it. A date and time would be good too. Just in case the buyer does something improper on the same day, but at a later time than the sale took place --OR-- the seller did something improper before that date/time.
Maybe I'm not paranoid enough, but I don't usually bother with a bill of sale when I sell a firearm. I usually don't even want to know the buyer's name. I just want to see a TX driver's license and DOB for proof of age. That's all I need to be a legal seller. True, the weapon may be used in a crime someday, and if originally registered to me the cops are going to come looking for me. Fine. I tell them I sold the gun. It may cause me a bit of a hassle being a suspect for awhile, but the authorities actually have to prove beyond a reasonable doubt that I committed a crime, not just that I once owned the gun. Since I am not generally anywhere near crime situations, and due to the nature of my work, I am not likely to be a suspect for very long.
So, I don't ever bother with a bill of sale. Mostly just because I hate unnecessary paperwork. Hope that doesn't bite me in the butt someday.