It's a good point. They need to work within the law. I'm not sure though that they sell in huge volumes. With number limits at every big box store, it's going to be a box here a box there. I'm not sure about the tax law end of it. They already paid sales tax if they bought it retail. I'm not sure how that figures into what they've sold it for.
One does pay sales tax when they buy a used car? Right?
You trade in a gun to buy another, someone buys your trade in and they pay sales tax.