Boxhagener Platz Flohmarkt
Berlin
Flea Markets
Second Hand
Friedrichshain is a great area, with lots of cafes and restaurants all around Boxhagener Platz. Known for its thriving underground and alternative sub culture, and for being a lot less gentrified than areas of Prenzlauer Berg and Mitte, the area welcomes a diverse bunch of people and also includes a few squats.
During the week there is a food market here but on Sundays the vintage gear, hand crafts and antiques come out. There are a lot of private sellers, which bumps up the price, but you are welcome to haggle for the best deal. Where you'll find the really cheap stuff is the stalls from Berliners selling off their old gear. If you are into books or vinyl, this market is one of the best to visit as the record stalls take up one whole stretch of street side shopping.
Of course there are food stalls and drinks available in the market or if you want to escape the bustle for a bit, hit one of the cafes on the streets around the platz. Most of them have good buffet breakfasts too.