Braderie de Lille is definitely one of the most interesting of its kind. And it's not because I'm a local

Actually people do come from all over France (all the way from Marseille for some) and from Europe too.
It's definitely worth it if the weather is good.
There are several areas. One is between "champ de mars" and the Deule (river). The people there will usually sell used items and what you'd find in garage sales.
Along the boulevards you will find a mix of professional sellers, antique resellers and a fairly large number of people who usually tour the marketplaces.
In the center of Lille, Grand Place and the Opera all the way down to the train station, you'll mainly find food and entertainement.
Side streets will also be filled with people trying to make extra money selling their old stuff.
Don't forget to try some "moules frites" (mussels and french fries) This is the local specialty. I don't know if they will still be allowed to do that this year but it is customary to build up large mounds made of mussels shells outside of restaurants, some being over 7 feet tall.
To give you an idea of size, there are roughly 10.000 vendors, occupying something like 100 kilometers of sidewalks. That's how huge it is.
in 2011 the Braderie had to be cut short due to catastrophic thunder storms
Here's how crowded it gets (this pic is taken on an evacuation route, so no vendors here, only thru traffic ... gives you the idea. Building at the back is Lille Flandres train station