I will sell my mini in short time and I do not know where I have to unregister the device at DJI.
The mini was meanwhile replaced by the mini2 and I dont want the buyer to get a drone which is
still dedicated to me somewhere.

Where do I have to take action?

You can't un-register.
The new owner will be added as a registered owner.

Hello there Angelone. Good day and thank you for the inquiry. The new owner of the said DJI drone just needs to create a DJI Account or use his or her DJI account for the new owner to be able to use the said DJI said Drone and once the new owner of the said drone creates a new DJI account he will be starting a new account and all of the past records will not be included on that account. Thank you.

The drone will be tied to the new users account when he opens the app and adds the drone but if he logs onto here it would not show in his profile.