The Clignancourt Flea Market is arguably Europe's largest, and stretches for blocks and blocks on the north side of the city just beyond the Périphérique, Paris's infamous "ring road".
What to buy: You can find absolutely anything here -- somewhere -- from 18th century antiques to African sculpture, and from old, worn-out toys to 19th century erotica. If you can imagine it, you can probably find it here.
What to pay: From nothing to thousands.
Paris has 3 main flea markets, and the most famous is Marché aux Puces de St-Ouen (Porte de Clignancourt.
This is really specialized in antiques, second hand goods and retro fashion.
The best days to visit are Sathurday and Sunday.
The Vanves Fleamarket is less known than the Saint Ouen one in the North, but very worth a visit.
Open Saturday & Sunday 7AM to 11PM.
What to buy: Old stuff.
At the Vanves Fleamarket there are some record dealers.
BTW Thanks to ForestqueenNYC for the scouting and the pictures.
What to buy: Records
What to pay: Higher prices than average