Your forgetting (for some reason) the 1/4 - 1/5 of the current (untaxed) economy that avoids paying social security, Medicare, and income tax entirely.
But anytime ANY of these people makes a purchase after the Fair Tax begins, that exchange is taxed.
Now ole Rob claims that almost all of the current "underground" (untaxed) purchases (he makes a incorrect assumption that the underground econmy is only drug dealers and prostitues!) are used goods and from pawn shops that would not be taxed, but that is wrong.
I never made that assertion, and haven't made a single reference to undergroung purchases, nor am I commenting on the potential black market that would result from a 30% tax on every retail sale.