My brother usually puts up booths at seasonal flea markets here in our area and in my experience, Marc is absolutely right in terms of Surplus items. They are easy to dispose of compared to the big ticket items, therefore, you would easily return your investment and get the products that you really want to sell.
You might want to consider a gimmick or a marketing scheme in terms of advertising your products at the flea market since the competition would also be stiff. Just like if the stall beside you has the same type of product. After all, in terms of eCommerce, how you present your product page is one way to generate more sales. It is not far off from actually selling at a flea market area. Buyers would always opt for what's pleasing to the eye.
I hope my insights helped. :)