No selling of “private” models. Private models are simply not published anywhere and available only on your account. Links to them given to other people will lead to “404” errors.
The purpose of private models most likely is to test stuff and send stuff to print over the cloud (eg via handy) without need to publish it.
Another use is temporarily making a model invisible. I had one where two users reported a problem printing because of a corrupted 3MF so I set it as private until I had a chance to sort it out so nobody else wasted their time trying to print it.
Seems to be a very handy option for private models you only want to print yourself, especially if you have several printers.
As opposed to slicing and sending the file to each printer on your network and saving the file on the sdcard. When updating the model you would need to re-send to every printer. It’s much easier to manage in the cloud.