As I wrote, for those who live near state borders I have no problem with asking to see a driver's license to prove same-state residency. Beyond that, you need to do nothing, and even that is (as I wrote) more than the law actually requires.
I don't live near a state border. However, I live in an area where regional accents can be rather strong, and if I were confronted with a buyer whose accent clearly indicated that "Yer not from 'round hyar, are you?" I suppose I would then ask to see a driver's license, too. The law is basically telling you to have your antennae out, and a "furrin' " accent would be one of those factors that could lead a seller to reasonably believe the buyer might not be eligible.
But the law is really addressing prohibited persons, not interstate sales. So a buyer has a driver's license from your state. How do you know he/she isn't an ex-convict, or that he/she doesn't have a protective order against him/her? If your state doesn't require a carry permit or FOID to even buy a handgun, there's no document available that says "I am not a convict," and there's no document available that says "This person is not subject to a restraining order."
I was not advocating "circumventing" anything, and I rather resent colbad's suggestion that I was advocating such. This is an open discussion, and I posted to clarify what the law actually REQUIRES. How far you wish to go beyond what the law requires is a matter of personal preference. Since the original question asked about bills of sale, it should be recognized that, while a BOS might make you feel more comfortable, it is not required by law and it WILL reduce the pool of potential buyers if you insist on one because I know I am not the only member of this forum whose position is that if I wanted a paper trail I'd just buy from an FFL. Not because I wish to circumvent any laws, but because the government is so much into our private business that I simply don't feel inclined to give up any more privacy than I am required by law to give up.