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.
The retailer will have a receipt from Vinny in L.A. saying he paid NRST or he didn't. The VA authorities will have to talk to the CA authorities to verify the receipt. If there was tax, Vinny could remit tax paid by some other buyer and claim it was paid by the VA buyer. If there was no tax, the chain goes even further.
On the other hand I can see how demanding receipts could uncover some smuggling.