In Japan, flea markets are often mistakenly called "Free Markets".
Earlier this month, Aki and I brought a bunch of our old clothes and other misc. items to our local flea market (at Maronie). We were all set up by 9am and that's when a flood of people started coming. IMHO (In My Humble Opinion), we had the best stuff there and also the cheapest. Aki was selling her DKNY dresses for a couple of bucks. I sold some never worn (with tags still on it) Gap t-shirts for 200 yen. By noon, we had sold about 20,000 yen ($200) worth of clothes and other "junk". It was a fun day and we didn't do too bad for just a few hours "work" on a sunny Saturday afternoon.
Another fun point about going to flea markets is that you get to bargain. In fact, it's probably the only place in Japan where people readily ask for discounts ("price down"). I was selling an old J-Crew vest which I loved for a dollar and the guy asked if I'd take 50 cents! He reminded me of my mom who will ask for a discount on something that costs a quarter (25 yen)! :)
Flea markets are quite popular here as they don't have garage sales like we do in the states. Temples or shrines are a popular place to have flea markets and they usually start very early in the morning.
If you haven't gone to a flea market yet, you should. There's something for everyone. Remember the old saying, "One man's junk is another man's treasure".