The gun you sell today can be resold tomorrow to an improper person. Do you want that to land on your doorstep? If no BOS is a requirement, they're just going to resell it tomorrow. If the deal is good enough to attract the attention of the no BOS group, someone else will buy it.
It's just insulating yourself should someone else bring trouble to your doorstep.
If it couldn't land in my lap, I would just ask to flash Indiana ID.
As most all of my interactions revolve around this site or a set group I am pretty easy going. No need to be critical with folks you know and you can find.
Not today ATF... Not today
I have an inventory records file. When I buy, I put the gun on the list. When I sell, I just record it as sold with the date, maybe a name. Mostly for my own peace of mind when I start looking for a gun that I forgot I sold 10 years ago!