I occasionally see guns for sale that look interesting. Then I will read the part where the seller requires the buyer to use an FFL, at the buyers expense, for a $200 gun. Or requires a FTF transaction when they live 200 miles from the nearest major city, and the seller refuses to meet in the middle, etc. I sometimes get curious about whether these guys actually manage to ever sell their guns.
But as you said, you can require anything you want, as either the seller or the buyer. If a business owner can require me to not be wearing underwear* in order to buy from them, they why would a private seller not be able to put any restrictions they want on a buyer?
* or shoes, or a gun, or a shirt, etc., etc.