Sorry it took so long to get back to you.
The possibilities for underground economy are endless. Any services people need can be delivered underground - plumber, locksmith, gunsmith, welder, automechanic, etc. Providing goods is a little tougher, but can be done as well - agricultural products, furniture, booze, clothing, etc. Essentially you just provide goods or services without a paper trail - no checks, preferably no FRNs, no receipts, no invoices, no taxes, no regulations, no names. Just exchanges of goods and services for money or other goods and services.
At the moment I am totally aboveground. I pay my taxes and play by the rules. But I am acquiring equipment and skills to be able to offer services underground if my job evaporates or becomes so laden with taxes and regs as to be worthless