i love sundays! most flea markets in berlin are held at the weekend, the majority of them on sundays. head for arkonaplatz (officially in mitte, but very close to prenzlauer berg) for vintage fashion, furniture, crockery and lots of diverse bric-à-brac. you'll find professional sellers and local people clearing out their closets alike. the market is small and prices are medium-range, but the selection is nice and there's been hardly a time when i left it empty-handed.
a favourite of mine is berlin artist thomas czimczik's stall with vintage shoes (some belts and bags thrown into the mix as well). i've bought many shoes there - he's very helpful and his prices are good! say hello from me when you go... :)
by the way: the close-by mauerpark flea market has been rather hyped these past few years. i used to like it in the beginning but now i tend to find it overcrowded, while the quality of goods is often poor. the last couple of times i've been, i was quite disappointed. so if you ask me: definitely arkonaplatz!
find the flea market on arkonaplatz, 10435 berlin, open sun 10-16h.