A seller or a buyer asking me for a Bill of Sale ends the transaction for me. Purely and simply, it's not legally required where I live, and what it absolutely DOES do is it puts your name and home address on a piece of paper, to someone you do -NOT- know, clearly disclosing the location of more firearms.
Know the law, follow the law. Where I live, my buyer needs to be a resident of the state I live in and 21 or over, and he mustn't give me any outward idea that he's prohibited.
If he ends up doing something illegal, or the next person down the line does something illegal, I am not liable and will not be held liable as long as I did my due diligence.
For those that feel they must stick to a Bill of Sale, that's all well and good. But disclose that information ahead of time. Don't waste someone's time with a meet-up and a deal and attempt to stick that foolishness at them.