"Plug-in is free and may not be used commercially or in conjunction with paid-for-services without the express written consent of the author of this plug-in!"
That would be non-GPL and we wouldn't be able to host it here on WordPress.org.
The GPL doesn't allow for extra terms like that. It is perfectly acceptable, under the GPL, to take somebody's plugin and sell it, as-is, or modified, or whatever. The GPL isn't about whether people can sell things or not, it's about freedom of the code and the passing on of the granted rights to that code to others.
Somebody can sell your GPL'd plugin, even bundled, but they can't do it under terms other than the GPL. Meaning that if you get a copy of that theme with your plugin, you can then turn around and give their theme away for free too.