Here's the way I see it. Is what you're doing adding/removing/changing content of the game that already exists? If so, then no, you should not be allowed to sell that mod. Sure, make a donation link or an OPTIONAL adfly link but don't try to sell me a couple new armor textures, or a handful of short side quests.
Are you basically making a new, fully fleshed out game with it's own story, 30+ hours worth of gameplay, a mix of you own assets and existing (or better yet completely your own), maybe some core gameplay changes. I don't see a problem with that being sold, but I also don't see that being done by a single developer in any reasonable time frame, and if you're willing to do all that work, you might be better off using a fully fleshed out engine with well built and well documented tools and just call it your own game.
The problem with a "Larian seal of approval" (as mentioned by another poster) is that I'd rather see them working on patches for this game, dlc, or a completely new game than spending all their time sifting through tons of "one new texture", "a couple side quests", "some added dialog", "MOAR RECIPES!!" type mods that I guarantee people would be submitting to try and earn a buck off of.
The best mods come from people who are passionate about the game or their vision of a story not from people trying to make money without leaving their chairs.
