If retailers are tracked at the wholesale level that would require coordination across state lines.
A person is not a retailer in the NRST sense unless they apply for certification and an ID as such. If you as a flea market seller purchase goods in one state, you pay the tax then and there on what you purchase to that seller. There is no tracking across state borders of taxed sales items to individuals.
Sell all you wish in a flea market and charge no tax to your heart's content. Just have your receipts ready if you are ever questioned by state authorities when selling goods in such flea markets.