Flea Market - Porte de Saint-Ouen

This vast flea market, founded in the late 19th century and said to be the Europe’s largest, has 2000-odd stalls grouped into nine marchés (market areas), each with its own speciality (antiques, cheap clothing, etc).
Open on Saturdays, Sundays and Mondays, from 9:30 AM to 6:00 PM, including public holidays.
Metro Porte de Clignancourt (line 4)

The Flea-Market of Montreuil
Established in the 19th century, this market is known for having good-quality, second-hand clothes and designer seconds.
The 500 stalls also sell engravings, jewellery, linen, crockery, old furniture and appliances.
It is open 7 AM to about 7 PM on saturday, Sunday and Monday.
Metro Porte de Montreuil (line 9)