OK folks, joking apart................ the question does have some validity?

This "underground economy" I suspect really means "black economy", using the terminology used when I studied economics way back in the last century (and milennium )

This is where people "trade" goods and services so as to remain off the "radar" of taxation authorities.

A crude example off the top of my head would be something like: "I drive your kids to school along with mine................you mow my lawn" In that scenario, you save time or the price of a taxi, and I save the cost of a gardener

Or, I repair your computer, and you fix my car ( for Morgana~ read "wine")

Good comment by westin........................ e-bay and similar "auction" sites, are the manifestations of this sort of economy. Otherwise there are specialist sites for product classes............. classic motorcycles, cars, boats and such.

I think that the confusion is due to misunderstanding this sort of activity as "underground" when it is really just a modern extension of the old "barter system"

As a bit of background, we have "capital gains tax" in the UK......... so I have this old antique (no! my wife is 7 years younger than me) and I sell it for $500,000............... but it was only worth $100,000 when I inherited it. That means I have to pay 40% on the $400,000 "profit" that I have made.

Another concept is "trading" ....... if I refurbish old cars, motorcycles and the like and sell them on e-bay or whatever, I stay off the "radar" as a trader?

Just remember it was the IRS that sent Capone to Joliet............ not the FBI