Please read through the following:
- http://codex.wordpress.org/License
- http://wordpress.org/about/license/
- http://en.wikipedia.org/wiki/GPL
- http://www.gnu.org/licenses/gpl.html
You can’t can sell it as software or development, although and/or you can also charge for maintanance and install.
Anyway, you should at least donate to the plugin developers, if you make money with their work somehow.
EDIT
I’m am sorry for my previous lines: I have to admit that GPL code can be sold, even if it’s not your code, althought I still recommend donating.