License plate short term for old and new car as possible?

Hello. I might want to sell my old car to a foreigner on “the green meadow” (Get there what it is still worth and don’t have a pimply “make-search”) Have to buy a used car from me at the same time (other registration district, but the same federal state). Would like to do it like this: register my old car yourself (because then it is certain that no one else can build an accident in my name or insurance…)! old and for my new “simultaneous” (or even if I am to log out at the registration office) register again. How do I have to proceed in order to drive my old car with my existing registration plates still to the dealer and then pick up my new car with these registration plates with the private seller (where they logically stay at the car)? I ask first here, as with my registration office, because most of the ones are the more expensive variant (previous short-term registration marks). However, I have heard that with my described variant it could also be possible somehow. BZW. MAY BE ON THE DAYS OF THE DECLARATION ONLY WITH THE DETAILS LEARNING THE DAY BETWEEN 24:00 AND THE DAY TO BE TAKEN ?? I have to hand over my registration papers and TÜV certificate to the buyer during the sale. I probably can’t get my new car on the same day with these registration plates, but only from the next day?I would therefore only have the fee for a desired purchase. sign to pay ??? In short: How should I/could I proceed? Thanks and VG