There have been a number of posts here that bluntly state to "trade cash for gun and be on your way."
I guess I'm unsure how it could be any more clear... a document signed by the seller on a date that predates or even post dates the period where he reports it stolen would be quite valuable if at a later date, in say 5 or 10 years or whatever, your gun turns up to be run on a check and comes up reported stolen from the seller you dealt with.
Could be a dozen malicious or accidental reasons... maybe he forgot he sold it, and is the victim of theft and simply reports them all stolen, including the one he sold you. Maybe he does it for insurance money. Perhaps he dies and his family finds a list of his serial numbers, finding the one you have missing from their list, and concludes it was stolen and reports it stolen. Unlimited reasons this could happen. With a serialized high theft item like a gun, it's prudent to have some records beyond a phone record.
Ever try to recall a phone record from a few years ago? Good luck. If you don't take measures to save those records, they are generally deleted and/or if you move companies they are gone or hard/costly to get. And try finding one unfamiliar phone number out of thousands spanning a few years... have fun with that exercise.
Or a document where the seller affirms the gun he's selling is legally his and not stolen... one piece of evidence affirming you were not intentionally purchasing stolen goods... a separate crime of knowingly receiving stolen goods. Your SAYING you didn't receive stolen goods won't be nearly as impactful as the seller having signed a document that states he was not selling you stolen goods, to help establish YOUR state of mind from a 3rd party.
Heck, even an email communication where you ask a few fundamental questions that establish it's a lawful purchase would go a long way to distance yourself from any allegations of knowing wrongdoing if years later there's some funny business and memories faded...
There you go again with the "what ifs." What IF you sign a bill of sale on guns that were already stolen? Are you not still out of money and a gun? What IF I buy a gun from a FFL, and then get my partner in crime to sell the gun to you, and then I report it stolen 5-15 years later after he has passed away?
I guess FTF sales should be illegal now too huh? What IF a criminal robs you in the middle of the Walmart's parking while you're doing the FTF sale? What IF the person signing the document just recently became a prohibited person? What IF you lose the BOS? Why are you even doing FTF sales to begin with? If you're that paranoid, and with all that can possibly go wrong, why not have all your firearms transferred through a FFL? You won't have to worry about any of the aforementioned "what ifs."
Why do you even own a firearm in the first place? What IF a criminal kills you with your own gun? What IF you accidently shoot yourself? What IF a criminal breaks into your home, steals your gun, uses it to kill someone, and then plants it back into your home? What if a kid gets ahold of your gun? What IF you try and defend yourself with a gun, and accidentally kill an innocent bystander?
What IF my What IFs are just as possible of happening, but equally not as plausible? I can cite and link to reports where FTF gun sellers were robbed, legal gun owners were killed by their own firearms, children who got ahold of firearms were killed. Legal firearm owners guns were stolen, used in a crime, and later replaced, etc... What IF you can not backup ANY of your WHAT IFs with actual cases of FTF gun buyers being scammed or falsely accused and charged with stealing or knowingly buying stolen property?
Millions upon hundreds of millions of FTF sales and firearm estates have had to have transferred from one owner to another. Please cite the cases were the phenonium you eluded to actually ever happen, because otherwise, the cases you are making sound no different than the cases the antis are making for gun control and against FTF sales period. I'm guessing that all your concerns aren't based on statistics and actual cases, but instead on what makes you "feel" better. There's nothing wrong with the latter, but I wish you all would stop speaking about it as if it's an proven epidemic.
Enough with the "What ifs"; now cite statistics and actual cases. The only cases I've heard of where someone was charged with possession of a stolen firearm was when the person was a felon, there was proof that they knew it was stolen, and/or it was a tacked on charge to another crime. Otherwise, even with a signed BOS, you can not know if the gun you're purchasing is a 3rd, 4th, 5th hand stolen gun or not. Again, if you're that paranoid and the laws on the books aren't good enough for you, then always go through an FFL...