I have used this company in the past to buy an apartment and am in the process of selling through them at the moment. http://www.adhoc-immo.de/ I found them very helpful and thorough. PM me if you want to know more.
I do not have any connection with them other than a business relationship.
I can second adhoc. Also just a satisfied customer. Many agents in Berlin came across as indifferent and lazy to me. Adelheid and Brian were always reachable and showed up when they said they would.