There are basically thousands of items that are great sellers at flea markets. The only thing left is to get a good supply on hand, then head to your nearest flea market. Before heading out, you should survey the flea markets in your area. If so, you also need to keep in mind that you do have to pay for your tables. Figure out how many tables you'll need and see if it is worth your time.
Timing is also important.
Weekday flea markets, as a rule, are not as busy as weekend. If at all possible, you should set up on weekends to get the most customers. Also, you will have to get up pretty early to get a table or two. Find out what time you can arrive at the earliest and be there so you get prime tables. Bring extra change in coins and bills.