I have nothing against modders getting financial compensation for their work, but selling mods can never work. The thing with mods is that they are being held together by spit and bubble gum, the smallest thing can break it. What about incompatibility between (paid) mods? What if a game updates and breaks the mod? 24 hours after downloading is not enough for testing, some people spend days just setting up mods before they even make a character. What if the problem only occurs 15 hours into the game and ends up breaking everything?

Then there is the unprofessional nature of modding: how do you know a modder is not using other people's content? All you get to see from the outside is a list of files and no one would be so stupid to keep the name of the stolen files unchanged. So a modder suspecting would have to buy every mod out there and check to make sure his files are not in there. But you can get a refund, right? Right, except it's in Steam vallet credits, not in actual money.

Valve's solution to the above: let the community sort it out.

And finally, the cut from Valve/Bethesda: 75%. You'd be better off just setting up a donate button on the Nexus, maybe you don't get that much from every user, but what you get is 100% yours.