How would a national retail sales tax apply to the black market? If I buy some dope from a guy’s trunk, is he going to collect tax & send it in? Or if I pay someone cash for produce he grew, what makes him send in the tax?
Black market monies would get taxed when spent, which is how a consumption/sales tax works. So while they wouldn’t be collecting tax, they would still be paying tax when they buy legitimate goods and services.
It also eliminates the need for money laundering operations, so there goes a whole category of crime that no longer needs investigated and prosecuted.
Crucial distinction: I said "black market money" and you said "black market".
If a guy sells dope, there is no way to tax that black market transaction (as you said) but once the drug dealer makes some profit, and buys himself some nice clothes, a nice car, and all kinds of things like that, all of his retail transactions (paid for with black market money) get taxed at a new rate (it wouldn't be a 5% sales tax: it would be more).
Currently, we all pay the 5% sales tax, and honest people pay income tax (black market profiteers don't). I say end the income tax and make everyone pay something like 15% on all retail sales. It balances out for everyone except the parasites who suddenly have skin in the game.
Beware of sales taxes, because they are just income taxes in sheep's clothing.
As soon as sales taxes become a major source of revenue the government will tax the "use" of products you didn't pay the sales tax for. Most states with a sales tax require you to pay that tax on anything you import into the state, or buy without paying their sales tax.
And then states go after services, wanting sales taxes to apply to services as well as goods, or wanting the sales taxes to apply to transactions where goods are provided with a service. That's just the back door to a tax on the gross receipts of self employed people.