Thank you Jan for the extended and interesting information.
In one point I am not sure if it is allowed. You said:
Also you must maintain the credit for that code as it wasn’t yours to begin with.
So if you modify any GPL code and sell it you can’t say to your customer “you may not distribute or modify what you have bought” or restrict the use and/or modification of that code in any way.
In this case I have to keep the message that it is powered by WordPress + Logo etc? (don’t worry, I love wordpress very much, it’s just because I want to see the possibilities which are possible).