Most markets in Barcelona have a pretty wide variety of brand-new, cheap-and-cheerful items for sale. True flea markets are a little harder to find, but the ones that do exist are gems:
Fira Brocanters Antique and Flea Market
Rambla del Mar
This one is a little market that packs a punch for the Flea Market fancier.
Located (beautifully) in the shadow of the statue of Christopher Columbus, stallholders set up every Saturday, Sunday and Public Holiday. Take your haggling skills; not only is it considered acceptable ettiquette but it´s also such a well-populated tourist market that you will need to fight to get a bargain.
http://www.encantsbcn.com/ (website in Catalan)
Pl. Glories Catalanes 8
This is Barcelona´s biggest flea market and is packed with goodies. Open on Saturdays, Mondays, Wednesdays and Fridays from 9am to 6pm, it is best to get there early for the best items.
Av. de Reus i Taulet, Sant Cugat
Not strictly in Barcelona, but rather in Sant Cugat (about half an hour by train or car), Mercantic need to be mentioned as a combination of market and shopping centre.
There are more than 80 permanent stores filled with second-hand goods and some surprisingly high quality antiques.
Although Mercantic is open from Tuesday to Saturday (9.30am to 8pm), it is best to go on Sundays when both the permanent stores and the weekly flea market trade from 9.3am to around 3pm.
Sant Antoni Book Market
Comte d´Urgell 1
Although it´s called a book market, the Sant Antoni flea market is a must-visit if you are a buff of retro items like vinyl records, old books, vintage clothes and magazines. Also has a great food hall for free tastings of local wines, cured meats and fresh fruit and veggies.
Only open on Sunday from 10am to 3pm.