Yeah...that's like the difficulties of collecting income taxes from illegal-drug dealers. In fact, there's not much difference, is there?
Sorry to be so stubborn about this, but I don't care about the lost revenue from the illegal drug market whether it is income tax or sales tax. All I am trying to point out is the difficulty of tracking merchandise and sales of that merchandise.
Granted, drug dealers have a much larger incentive to hide their production, smuggling, and sales, but the basic principle applies to any merchanidise. That principle is that merchandise can be produced, smuggled and sold without the government knowing about it, except for the small fraction that they are lucky enough to find.