You have no rights, unless explicitly given, to sell something you made using the work of someone else. You did not make the Divinity Engine, you did not make the tools. You have no claim whatsoever to monetary compensation simply for using tools freely provided by the developers.
Modding is not a job. You are not suddenly a developer or game designer simply because you can use the modding tools that the actual developers created. If you don't like the idea that you don't actually deserve money for making a mod, then don't make the mod. There are plenty of folks out there that make mods simply for its own sake.