What do you mean by "make a site and sell it?". What type of sites are you making exactly? If you’re just building sites for your clients, or if you’re creating something that sells products, etc, you’re free to do what you want.
However, I wouldn’t recommend selling Themes that you’ve purchased from here, StudioPress or any other WordPress site and selling them packaged as your own. Most people in the WordPress Community don’t do that (they create themes themselves from scratch). The few that do take other designers themes and attempt to sell theme usually end up stopping, since it is highly frowned upon…Not saying you would do something like that, but just in case you’re wondering.
If you’re interested in selling your own Themes, I would recommend starting from Scratch using the Genesis Sample Theme so then you won’t have to worry about using anyones Code, Images, Design, etc.
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