It's Not Your Money, Its Ours
The "underground" economy has been around a long, long time. It's probably the oldest economic system in the world. It is the in-kind commercial exchange system--the "I will fix your leaky spouting if you repair my car" economy. No money exchanging hands. No bank accounts. No tax (except GST, of course, if you have to purchase parts and materials).
This in-kind system, however, is very, very inefficient. Money is required to make the exchanges easy, efficient, and equitable. But the underground economy has a monetized solution. Pay cash. The benefit--no tax. And that is the biggie.