I have been on the reciving end of a situation like this. If you are the person who bought the firearm new you will be the first person they come looking for. You need to keep some type of information as to who you sold it to.
Me too and you don't need to keep any information as to who you sold it to. All I told them was that I had sold it, they (BATF) said thanks and left....end of story. As someone said you can make up a bill of sale so why would that have any legal merit. In this day of identity theft no stranger selling me a gun FTF is going to get any personal info and/or a copy of my drivers license.
And just to play devils advocate, what info should I as a buyer get from the seller? How do I know he didn't steal the gun. Shouldn't he give me a copy of his original bill of sale he received when he bought the gun to prove it's legal?