If you want to do it bigger, you can buy new merchandise from wholesalers in bulk at very low prices, just do a Google search. This new merchandise can then be sold for a profit, but make sure that you're selling it at lower than retail or no one will want to buy it. They go to the flea market to get things cheaper than they would elsewhere, therefore you should make it available at a cheaper price but still high enough to put some money in your pocket.
Flea market rates run anywhere from free to $25 a day for the larger and nicer ones. I would say that $10 a day is about the average cost to set up shop. Whatever items don't sell at the market you can put up on eBay or craigslist at the same or lower prices, and hopefully you can get all this stuff off your hands. The idea is to get rid of your inventory as fast as possible; you don't want to be stuck with a bunch of stuff that doesn't sell. Sell it for the price you bought it for just to break even if this is the case.
Go out there and make some money for yourself! The best part of being a flea market vendor is that it is part time, you are your own boss, and you get the satisfaction of making this money yourself in a fun and interesting environment. You will meet lots of interesting people.