Selling a car, then logging off?

hello, what happens if I sell a registered and insured car on Saturday or Sunday, make the number plates (because dealers collect red signs), correctly hold the time, date in the purchase contract and shortly after “something happens”, accident or other. Is the car still registered and insured on me? I can not inform you until Monday e.g. registration office and insurance. Will only be safe there. Bzw. what does the buyer get everything? Tüv Bes registration and letter, that’s it? Registration certificate I know that I need to log out then? Don’t you also need the letter to log out? how is that supposed to work?