Buying a car – How do I do it with the registration? (Residence – Berlin)

Hello together, I have a problem and need advice now. Next weekend I buy my first car. (visit etc. Everything is done, he will fix some defects and then I will take the next WE with me.) Now it’s about the following: Sunday we conclude the contract, until then the car is still registered to the seller. Insurance etc. Is not the problem with the ebv number. But if the seller unsubscribes the car on Monday, and at the time then at my home v or the door is standing (public parking lot – no private or similar ) then it stands from time x unannounced, and that a pkw yes not allowed. But since I live in Berlin it will be a bit more problematic, since you need an appointment here at the registration office, without appointment nothing goes at all in Berlin. Therefore I call Monday a registration service that allows my car within 1-5 days (normal duration 24-48h according to homepage, so plan a that it is ready within 5 days) Jet zt is the question, what do I do with the car? I can’t leave it that way, is not registered… the seller has no property or the like. Where he can leave it until registration. Therefore, my question, how do I proceed best now? Or can I leave it there the registration is in progress? I thank you already in advance, for every useful advice