I guess you miss the notion of liability. Go try to buy dynamite,
certain chemicals or electric parts that can only be sold to a
qualified person with a license. They would be silly to sell to the
average non-qualified, non-licensed person.


That's completely different.

No one needs a license to buy stuff from Grainger. You may need a
electricians license to install a blower motor in some locations, but
buying that motor is not going to cause an explosion like dynamite or do
other harmful stuff like the chemicals. Grainger also sells a lot of
tools and other stuff anyon can use.