Most states require you to charge, collect, & remit sales tax on items SOLD in their state. This could also involve sales tax for the county also. A good place to start is to go to the website of the Revenue Dept for whatever State you're selling in. They should have info on what you need to do, if not, then a phone call should answer your questions.
At the flea market, adjust your prices to include the sales tax, and round off to nearest dollar to make a simple. Also, figure approx. how many XXL & XXXL you'll sell, and just add that to your price, so that all your shirts of the same type sell for the same price (which includes tax, and something extra to cover XXL & XXXL). That makes it easy so when a customer walks up to your table, you sign will say $XX.00 per shirt. Also, you might consider offering discounts for multiple quantities; customers like a "deal" and will tend to buy two if there's some type of discount.