I have actually seen a FairTax oppenent claim that the FairTax will not get the blackmarket drug dealers and the such to pay taxes because they are not going to charge the tax on the crack, cocaine, heroine, etc... that they sell!
Which is a legitimate point. The Fairtax does not capture the black market in anyway shape or form. The Fairtax just taxes legal purchases, which is exactly the same situation we have today. The net gain is zero if you really understand the argument, which I will not be holding my breath on.