Try this exercise:
On your computer, type up a bill of sale. Fill out info for an imaginary buyer.
Fred Smith at 123 Imagination Hwy Nowheresville, KY.
Dream up a gun if you want. Mr Smith bought my Kia Sorrento 9mm Serial Number 0001 (very rare, vintage, classic).
Now sign it with your weak hand. Or deliberately alter your handwriting.
Congratulations! Now you've got "proof" that you sold your Kia, in case Mr. Smith uses it to murder your wife later this week. Terrible tragedy it was.
Every BOS you keep, every one you fill out, every one you encourage others to complete, carries all the legal weight of the one you just invented.
You, as a private citizen, do not have the authority to create a "paper trail" for a gun. The only person who can make a paper trail is an FFL dealer. Your personal records are not legal documents and will not shield you from prosecution or suspicion in the case that a gun you bought from a dealer and later resold is used in a crime. However, keeping your mouth shut and not making statements to the police without consulting a lawyer will do a great deal to shield you from prosecution, if not suspicion. I consider this to be of much more value than a bill of sale.