As I mentioned, I went to a great flea market this past weekend. It's sort of "curated" and the vendors have to be approved by the Load of Fun group before they're allowed to sell. Load of Fun wants to make sure that the monthly market doesn't get trashy. I got some great things, and would have gotten more if I could have found an ATM.
When I was travelling to Paris on a regular basis, I would always try and visit the amazing marché aux puces or flea markets. Some flea markets here in the US have a bad name as a place to get rid of stolen goods, or sell multiples of white tube socks. I have been to flea markets in London and Rome, as well as some of the really fun ones in Los Angeles but I love the French ones the most.
Clignancourt is the most well-known of the Paris markets, but there are two other large markets, as well as smaller brocante (second-hand) markets that pop up around the city on an irregular basis. Bargains at these markets are less frequent than in the past, but there are always a ton of interesting things that you won't find in the States.
Lile also has a huge annual sale, La Grande Braderie de Lille, which took place the first two days in September. This market is much older than Clingancourt, dating to medieval times. Any resident of Lile can find a spot and set up shop. You are always expected to bargain at these markets, but be polite about it. Now's not really the time to be looking for bargains, especially with the pitiful exchange rate for the US dollar.
Photos of the Lile Market: The Guardian UK