This smells like a same fuck-up Blizzard had with Diablo III market. I mean, giving people free rain to set prices as they see fit and sell all kind of material without strong control? Yeah, right...
Usually it gets exploited in some way and usually those that think that they have looked at the exploit problem from ''all sides'' get a nice surprise when it happens. It will be messy. How can you prove yours virtual goods are your own? I can only imagine the nightmare of having two identical mods where one is cheaper than other, but from different sources and when reports start flying back and forward.

At least devs can choose to allow this option for their game. I hope those smart ones will say no. What happens if someone makes a mod where your character can look like Neo from Matrix and then sells like 2000 of those for some nice price? Is Warner Bros gonna sue him for taking money on account of their copyright material?
What happens in those situations? Someone gonna look through all those mods and get approval, make selections? I don't think so. It would be a job of nightmares.