I do not find it helpful in cases of protection and rights to look at anything other than individuals.
then let's agree to disagree. Here it's ok to own as many cars as you like, the restrictions are on driving
actually, afaik, it varies by state too; in any case you are going to have a difficulty buying a new car if you don't have a license (and i dare you to prove me wrong on that empirically).
and anyway, yeah-- the assumption with a car is that if you buy it, you probably need to drive it. same with guns: you buy it, you probably need or might sometime need to fire it (although i'm perfectly fine with people buying a gun to be used purely as a decoration, providing its firing mechanism will be disabled), for which you will need a license. So yeah, I think it is very logical to check that someone has such a license when the potentially dangerous item is sold.