A very helpful person at the Strassenverkehrsamt suggested that the buyer secure temporary license plates. To obtain these, the buyer required only a photo of the Ausweis/registration. I have gotten the Invalid/Ungültig stamp now, (and kept a photocopy) so I can give this to the buyer. I will remove the plates and after he signs two copies of the TCS contract I shared with him, he will take possession of the car. Now the process is clear. Thanks for those who helped here! Grüsse!