As for the automobile, California has long had more stringent air pollution standards for autos than the rest of the country so they could ban the sale of an auto that didn't meet the standards. As to the waiting period, I think that is a perfect example of how the feds do
not occupy the field of firearms sales. They only require a background check which doesn't provide a so-called "cooling off" period to prevent rash homicides or suicides.
Federal preemption can get to be fairly complex and a lot of good lawyers never deal with it because of the nature of their practice. I had occasion to deal with it a couple of times years ago because of my duties at the time. My first post, though, practically drained everything I remember about the topic.
Oh, one minor addition -- the courts are more likely to say no preemption if a state is exercising its traditional police powers, but there's a lot of grey there.