In fact, you have no basis for the statement at all since it is just as likely as not that the John & drug buyer paid little or no in the way of income taxes.You have no basis for that statement. Unless these people are using counterfeit money, all money in the underground market can be traced to legitimate sources.
One portion of my post was about using stolen money to purchase drugs - which is pretty common.
It isn't "counterfeit", but the thief is not a legitimate source for the money - or perhaps you think he is so you can try to further your flawed argument?
The current system gets very little of the money "going in". The FairTax clearly does a better job of capturing tax revenue from not only drug dealers but the underground economy period.
Don't forget that illegal aliens are also part of the underground economy and the income/payroll tax system captures very little in the way of tax revenue from them (either) "going in". And keep in mind there are millions and millions of foreign visitors who likewise pay no tax at present and similarly have little in the way of tax revenue captured.