I don't mean any disrespect to your idea but just want to share my experience. For years I dreamed of going to Paris and to the flea markets-I planned my trip around spending a day there. My DH promised me the bag of my choice for an anniversary gift. We went to the big market and I was never so disappointed in my life. Everything was so expensive and it didn't seem like a flea market at all-but a very expensive antique mall that kept going and going. I saw a handful of vintage bags but they were so rude about letting you look or touch anything-I couldn't imagine how I could even buy one.
I left the market after a full day with a handful of vintage post cards. It was such a waste of the day except for a great lunch where we met some really nice tourists.
It seemed to me if you are looking for a bag, Chanel has a pretty good selection there as compared to the boutiques here, but I l would focus on a local designer that you won't find much of in the US. We were there about two years ago, and I noticed that so many things in Paris in the tourist section now saw made in China, so buyer beware....