Maybe a bit too late for the OP but maybe of interest for other readers...
Remar 2nd Hand Shop
This is a small street very close to the entrance of the Zoo.
Most of the offered stuff is used furmiture, but they also sell electrical appliances.
(this is actually a christian charity organisation!)
(check Impressum for opening hours)
This may be the store DMcinDE referred to. They sell stuff from household clearances.
Very varying quality (and varying prices); they have some really odd and freaky stuff.
Worth visiting for the impressions alone.
(near the jewish museum / Konstablerwache)
Used Audio / HiFi / TV / PA stuff.
There is also another used HiFi shop in Sachsenhausen
(near the mattress shop).