Hi,
Why don't you check out zwansversteigerung? Its free, you are buying at the Amtsgericht (court) and you can get real bargains there.
I don't know about Berlin, but done it in other - more attractive in my eyes - locations, didn't pay fees, you get a valuation and even if sometimes you have to refurbish you can get real value for money.
Check it out, and leave all brokers behind but it is not online so you would need physical presence in the Courthouse.
Look in google for "Frankfurt Amtsgericht", click on "zwangsversteigerungen" and download a offer or two so you would get a feeling for it.
And the Auctionhouses, the commercial ones, usually offer you either overpriced or simply the **** of the market - be aware! This is not the UK, and there are hardly serious stuff on sale.
Enjoy!