The flea market at Boxhagener Platz in Friedrichshain can be great, when the weather is fine. You’ll find a lot of vinyl and a lot more junk, but there is also some good fashion. If you’re lucky, it’s cheap, if not, it sucks. There are a lot of private sellers so bargaining is obligatory. Saturdays there is also a nice farmer’s market, where you can buy a lot of fresh goods.