This type of thread seems to pop up periodically, on TFL and on other "gun" forums. As always, it seems there are two general groups -- 1) Those who are rigorous about wanting private sales/purchases to be "off paper" (meaning neither party gets any documentation and there is no paper trail); and 2) those who feel that they have some protection, whether buying or selling, by having both the buyer and seller sign a bill of sale, and both parties keep a copy. I understand and I sympathize with both views; each has arguments in its favor. I think each of us has to find his/her own comfort level ... perhaps more when selling than when buying.
Nobody wants to sell a gun and years later have the police knock on the door because "your" gun (that you sold years ago) has been recovered at a crime scene. Yet the fact is that you can comply with all applicable laws (in most states) and have this happen. Unless your state requires a background check for private purchases (mine does), the law doesn't require that a seller perform any "due diligence" at all. The federal law says we can't sell a firearm to someone we know or have reason to believe is a prohibited person. The law doesn't require you to ask if the person is prohibited. The law doesn't require you to verify that the buyer lives in your state. I think it's safe to assume that most of us would probably perform some perfunctory level of verification before selling to a private buyer, but that's where it starts to get muddy.
Just in this thread we've seen members say they would insist on a written bill of sale, others who want to write down the buyers name and address (and probably his driver's license or carry permit number), and still others who would walk away from a buy if the seller tried to write down their personal information. Yet, except in a few states, any of these approaches would comply with the law.
Personally, I don't have a choice because of my state's laws. The very first pistol I bought was from a private seller, and he had to submit the sale through the state police for approval. Since then, I've always bought through an FFL. Being a conspiracy theorist, I believe that we might see a day when the government tries to confiscate privately-held firearms, and for that reason I would love to able to legally buy a gun or two "off paper." I can't as long as I live here, but if I ever find a way to relocate to a "free" state, that will be on my agenda.
And yet I admit to being a hypocrite, because I certainly don't want any gun I might sell to be used in a crime. For that reason, although I would buy privately if I could, I won't sell privately. I don't like to sell guns, but over the years I have had to sell a few, and I have always done so through an FFL, usually on consignment. That creates a hard document showing that when the firearm left my possession and control, it went to a licensed FFL. Anything that happens after that is on him.
It's a matter of differing levels of what makes you comfortable, and I don't think we should criticize others if their personal comfort level doesn't happen to agree with ours. As long as it's within the law -- it's legal.