As I am also looking for a flat in Berlin I can only say: At least you got a response. It's nearly impossible to find anything below 20€/m² cold rent and some people are trying to (illegally) sell on old, cheap contracts for 6000€ and more.
I'm not sure if this is what gp meant, but:
In some cases the current tenant (who is looking to move out) offers you the landlord's contact details if you agree to buy their kitchen for way too much money. This works because for a landlord looking for new tenants is a lot of hassle so they're likely to take you if you contact them.