Legal to sell interfaces?

[Eloa]

I'm working on a project right now. A project that requires great depth into the game and a good deal of time. So much that I would feel time wasted just giving away such valuable information.

So I was wondering if anyone had anything on the legality of selling your interfaces and the different aspects of it. Of course it would have to be your own code, images, and all that I know. But is there anything preventing us from selling our one offs?