What are the laws concerning developing WordPress sites using other developers' plugins and then selling the sites?
Please read through the following:
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.
Under GPL you CAN resell the software.
You may charge a fee for the physical act of transferring a copy, and you may at your option offer warranty protection in exchange for a fee. GPLv2 Section 1
The main restriction is if you do sell (or give away) you must:
1) allow others to resell your works
2) provide them with the original and modified source under GPL.
3) Don't claim ownership/copyright of the original code and clearly start where (if anywhere) you modified it
Note that although WordPress.org requires all plugins in the WordPress.org repo to be GPL that doesn't mean it's true for plugins found outside of WordPress.org. It is YOUR responsibility to know the license. The biggest example is a lot of themes will license there PHP template files as GPL but not the CSS, JS, or Image files. Also some plugins will be GPL compatible but not GPL
WordPress.com uses plugins made by others to enhance the site and make a profit. Nothing says you can't make money just that if you distribute the code at all you have to allow others to do the same. Part of CNN.com runs WordPress has made and modified code that will never see because they don't redistribute it. Because the plugin authors have redistribute their code that changes everything.
Really it also comes down to ethics. In most cases credit/donations are nice. I also think it depends if you block the end-user from the source or allow them to see that it's WordPress and which plugins are used or are trying to create a closed source system.
I'm not a lawyer I suggest that you talk to one before starting a business reselling GPL Code. My advice is purely from my own knowledge and understanding.