GPL says you can sell the software for whatever price you want too. I could take any piece of sodtware licensed under GPLv3, bundle it up, charge people for that software (while telling them that what I am selling them is GPLv3 software and informing them that they can request a copy of the source code including any modifications that I have made to it). That’s totally fine. But why would someone pay for the software if the seller is not adding any value to to product? cough windows app store cough