On the one hand I do like the idea of being able to make a few bucks from work. On the other hand mods get away with a lot of stuff because it's free, although people nowadays sell games on their alpha stage so maybe modders can get away with selling incomplete, buggy products. But seriously, ban copyrighted content; the first page is full of things from other games and you shouldn't be able to sell things that technically aren't yours.
Why mod isn't a knockoff, it's an homage. I'm also not charging for it but rather giving it away free for a mandatory tip.