For the rest of time, I can be held accountable for the whereabouts of any gun that I own ar have ever owned, in the event that it was ever used in a crime. If it's ever traced to me, I best have hard evidence of what I did with it.
In my few private gun transactions, I require several valid forms of ID, which must be recorded on a bill of sale. That's made clear before meeting. If that's not agreeable, then it's not gonna happen. Simple as that.
Like the cpx that sccy replaced. I don't know what they did with it. But I have their paperwork stating both serial numbers. I keep that. And I have a bill of sale, which the buyer completed, when I sold the replacement. I keep that. It's a paper trail, for my own protection.
All states have different laws concerning gun sales, and each requires its own personal diligence. There's usually the option of meeting at a LGS, and having them perform a formal FFL transfer and background check, for a fee.
Armslist is a valuable resource when used wisely. Beware of its potential. Educate. What if it goes away?