Bill of sale is just prudence for both party's- You get stopped by the cops and as is so routine now they ask if there are any guns in the car.
Your being silly and say Yes- they go run the serial numbers and oops its stolen- better have a bill of sale
Same thing goes for the buyer using it unlawfully and (being silly) leaving it at the crime scene. THen you the buyer gets a late night visit, your dog gets killed etc etc the usual and the cops ask YOU where you were whenever and what about the pistola -Nice to have that Bill of sale.
The BOS will do NOTHING in either of the above instances, since it's not notarized it's therefore not a legal document - period. Unfortunately, there's lots of folks around that THINK they know what they're doing....that don't. So they make folks jump through unnecessary (and useless) hoops. When someone tells you they want a BOS or to copy your DL/CHL, tell them to pound sand and make a note to yourself for future reference that at best, they're fools.