Most sellers want a BOS to “prove” that they followed the law, that the person they sold to was a resident of the state and of age. It is easy to think this way, but it tramples our entire legal system and form of government. Following the law is what is required, going further to prove that you followed the law indicates that you’re okay with having to prove that you followed the law, when you should instead insist that the state prove that you broke it. Just one BOS makes no difference, but the trend is bad for our form of government.
A seller may also want a BOS because they fear that the gun may someday be recovered at a crime scene and that they will then be a suspect in that crime. This fear is unfounded.
Further to the two paragraphs above, the BOS proves only that you have a piece of paper, it doesn’t prove that you actually sold the gun. In the first case it is evidence that you claim to have checked the buyer out, but it is not proof. In the second the police will appreciate having another name to explore, but that isn’t what moves them off of you...they were really never on you. The paranoid seller will say that even if it isn’t proof, at least it is evidence and that’s better than nothing, but they are wrong it is exactly the same as nothing.