[Poohbah]

A sales tax does not capture the underground economy. Noone pays tax on a drug transaction. Neither the drug seller nor the drug buyer.

When the drug seller pays for other goods, such as cars, clothes, et cetera, the proceeds of his transaction get taxed at that point.

[SolidSupplySide]

When the drug seller pays for other goods, such as cars, clothes, et cetera, the proceeds of his transaction get taxed at that point.

That is true regardless of whether we have a sales tax or an income tax.

You are describing a transaction in the legitimate economy. Transactions in the underground economy such as drug deals don't get taxed by either a sales tax or an income tax.