That's an extremly thin ice you're working on.
Mojang permits modifications of Minecraft for non-commercial use only.
Their Terms Of Use seem to disagree with that assessment:
QuotePlugins 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.