The biggest problem is that US copyright law is already really, really shaky on this subject. Bethesda has just royally stepped in it here, because up until now, mod creators couldn't charge for their work without facing a suit for infringing on the IP and using the assets that belong to the game and studio.
NOW, however, Eddie in East Prussia can open his own Mod Resale shop. He can undercut both Steam and whatever cut Bethesda is taking, and charge a smaller pricetag.
Steam and Bethesda can sue Eddie, but now Eddie's got the law of precedence on his side. Bethesda's going to have to prove their assets are a license, and that Steam isn't just a storefront.
That sounds like not much fun.
Fair use laws in Europe are going to giggle at any attempts to shut down third party sites that also sell Skyrim mods.