I would think that the TCA would have a problem with people selling flea-market-style in the parking lots. Pre-aranged meets to exchange items/payments are one thing (and is done all the time), but setting up flea-market-style would be another (after all, the TCA is paying for the venue). If you are going to be at York all 3 days, you could always get a table in one of the member halls. They are only $18.
Andy