there´s spacehall for cd´s and electronic stuff and they have
opened up a second quartier for 2nd hands off all kind.
(dont know the name atm. but you´ll find it. otherwise ask
for spacehall on the street.)
they´re vinyl enthusiasts and i bet you´re gonna find
some good stuff.
kreuzberg is anyways a good spot...
and try kastanienallee. there a a few different recordshops
on one street. might be a little overpriced due to tourists
but at zionskirchplatz you will find a little place packed with
2nd hand vinyl on affordable prices.
if you need any further tips or help, feel free to pm me.
have fun.