> You don't even need to be the copyright owner to sell GPL software
You do, if sell means also allowing another vendor to use your software as a part of their proprietary software.
GPL alone, if you're not the copyright owner, would not allow you to sell software in this way. But if you are the copyright owner, you can bypass GPL and sell a proprietary license to the vendor saying "you can use this software in your proprietary product if you pay me $XYZ".
Red Hat, SUSE, Canonical, and myself, among many, "sell" GPL software that are used by others as part of their proprietary products.
Each of those, and the customers, are required to make the source they received (not their own proprietary source), including changes, available to those that receive the software (thus indirectly back to the public and the authors). AGPL is similar but includes the case of providing sources for products used in Internet services.
For anyone who thinks I'm talking out of my ass, consider Qt.
If you don't own the Qt copyright, you cannot sell Qt to a company who wants to use it to build a proprietary product - since Qt is GPL licensed, using it in a proprietary product would not be permitted.
But since The Qt Company owns the copyright, they can sell their own software to companies that want to build a proprietary product with it - they just sell it under a proprietary license, specifically tailored for this. The fact that it was previously released under GPL is irrelevant, because the copyright holder can license it under as many licenses as they want.
If you don't own the copyright to it, you can only """sell""" GPL software in a manner in which local DVD shops sold pirated movies back in the early 2000s.
You do, if sell means also allowing another vendor to use your software as a part of their proprietary software.
GPL alone, if you're not the copyright owner, would not allow you to sell software in this way. But if you are the copyright owner, you can bypass GPL and sell a proprietary license to the vendor saying "you can use this software in your proprietary product if you pay me $XYZ".