I agree with Razot, but I also agree there could be improvement.
Basically, everything Razot just said, this will grow mods out rather than diminish them. Imagine a world where no one could make money from their mods. Hey, you know that game counter strike? NEVER EXISTS, team fortress? NEVER EXISTS, garrys mod? NEVER EXISTS. Killing floor? NEVER EXISTS. etc. etc. etc.
People deserve money for their work and might convince more professional programmers to start developing cheap, awesome mods for games.
That said, the biggest issue I see is the fact that the market is LAWLESS atm, ie: Stealing is a big problem. People can steal someone elses mod and sell it as their own and currently there isn't a whole lot anyone can do about it. Placing restrictions would be very difficult to contain AND that'd make mods a real "legal thing" which would defeat the whole purpose of modding by getting rid of creative license. The other problem is the fact that if someone sells a mod the mod could be shit or break your game and there isn't too much you can do about that.
The solution to the second thing is to have a refund time, which valve currently has (though it's only 24 hours). The solution to the second thing I believe is to have a commission fee. IE: If you want to sell a mod you first have to pay a certain amount to the company up front, say, $300 or something. This stops all the people who release cheap and bad mods and also deters people from stealing mods without knowing they're gonna get a return.