Black market already exist due to rampant government taxes and laws that make certain things illegal. Most just never touch it unless they are involved or need to get items. currently the three biggest black markets are for illicit drugs, next is prostitution, then comes gambling and guns, you sat wait all those are illegal not a tax thing… there is trade for slaves there is trade for organs. There is trade for chemicals mostly precursors for drugs but some are for other things such as explosives. Any of you who live in port areas make sure you know the tax exempt area… inside 100% for sure are military containers full of just in case goods.. Now many places you can pay for cash for items such as clothes, food, many ethic communities have such places.
Chop shop cars get sold, passport traded, drivers license paid for , scrap metal bought and sold, air line points traded. you name it it is there, if you are going to bug in you should know some people and the places and markings used to id places that have this trade. Will be infinitely useful later. Hence know your local thugs and criminals. In china town you can get brand name electronics 50% off and they have 100+ inch plasma tvs
There are groups who target the docks organized crime whole containers leave and the insurance pays no one says anything and never any video evidence.
there is a barter web site in my local city where you can trade work for work….technically illegal because no taxes. We have people who now pay in silver currency $5 dollar coins at going rate of $20 you pay only in face value a amount totaled by actual value and pay the tax in fiat currency from the coin face value.
And i do live in north America