The regular and ever-popular Barcelona Flea Market is taking place next at Plaça Blanquerna in Raval (between Drassanes and Avinguda Paral·lel) from 11am to 7pm on April 29th. There you can find second-hand clothes, books, records, accessories as well as a few knick-knacks.
If you'd rather be a seller than a buyer, no problem. Just send a mail to email@example.com telling them what kind of things you'd like to sell (has to be second-hand, no manufactured or hand-crafted stuff), and they'll get back to you to let you know whether you've got yourself a stall. Just a word of warning, this special outdoor event is a bit more pricey than the normal Flea Market, so having a stall will cost €20 (although they can be shared).