Flea markets are organized markets for trading low-value and secondhand goods. Flea markets can provide you with a fantastic place to buy low and sell high. It's not uncommon to purchase an item such as a dress for $2 and resell it for $30 on eBay. As with all forms of product sourcing, however, you must ensure that you can make a profit on the items you buy at a flea market.
Here's a guide at sourcing and selling popular flea market items such as antiques.
Specialist knowledge in a hobby item will help, as will experience. Just remember not to spend more money than you can afford to lose, especially when you are just starting out.