I’m not an expert, but doesn’t the lower contain the components necessary for an AR15 to be converted to full-auto? Seems to me that would make for a sizable black market for just lowers.
Not the types you or I would purchase from the local gun shop. You are correct that the components for 'full auto' go into the lower receiver, but those parts aren't in civilian versions of the rifle. I've heard of a part known as the "drop in auto sear" which is supposed to be something that one can easily 'drop' into the lower to achieve that functionality. I believe one can even locate the plans for one out on the internet(Google can help there if one is so interested). I don't know if such a thing is for real or a myth.