Sure - you can't stop regular people from making small purchases from other regular people.

But the INSTANT that a regular person becomes a regular small business, the tax authority can and likely will ask to see their records. The INSTANT someone buys a car, the government will be interested in knowing how they paid for it.

Even in the most extreme situation, the government can and will demand taxes to be paid in its preferred currency. How are you gonna get hold of a substantial amount of dollars or remnibi or whatever without getting caught?

This stuff is enforced and no amount of math can beat thousands of years of the state figuring out how to collect taxes.