YOU are the one claiming a positive (That a BOS for a gun sale serves a useful purpose) so it should be easy to prove. I can't prove a negative. If I said clicking my heals three time makes any used gun more accurate I should be able to prove it.
A BOS for a gun purchase is NOT required and gives personal information (based on the way many do them) to an unknown party that has the potential to do HARM to the person supplying the information. It also provides NOTHING that can be verified in other ways as you yourself have already admitted. THOSE are the FACTS.
Doing them is idiotic. THAT is an opinion.
You should charge city prices.
And, I am not saying they are required. You know that. Yet you keep repeating that they are NOT required as if someone is telling you they are required. Everyone understands that. No one in this thread has said they are required. Yet you keep saying that over and over and over. If you want to go on believing that a bill of sale has no legal relevance, then go for it. You might as well also say that a written contract between two people is also not relevant in court if the contract relates to guns. You are trying to convert your dislike of bills of sale into a general statement of law about the admissibility of bills of sale in court.