Free: Contests & Raffles.
I once sold a handgun to a guy i work with. I went to the states website (i think) and downloded a gun transfer form. Then i had him sign it gave him a copy and mailed it to Olympia.
I have sold a lot of guns that I bought new. I have never done a bill of sale for any of them. I would not lose a second of sleep selling to anyone that I thought could legally own a gun. If its used in a crime and you are worried about getting in trouble about it then you worry to much.
If an AR stripped lower is sold as a pistol lower, it can never be used legally as a rifle lower. You stick a buttstock on it, and it's in violation of Federal Law. Again a violation of Federal Law. For that reason, most, if not all manufacturers of AR pistols are plainly stamping the lower "Pistol Only".