That's an extremly thin ice you're working on.
Mojang permits modifications of Minecraft for non-commercial use only.
Plugins for the game also belong to you and you can do whatever you want with them, including selling them for money.
Now, one might try and debate what constitutes a "plugin", but it's hard to arrive at a definition that doesn't mean "mod". Most people wouldn't think to nor care to sell their work - but Mojang's license seems to explicitly, rather than implicitly, allow for it.