To be fair, even Turbosquid seem to suffer from this kind of problem, for example I have seen several Poser meshes for sale via Turbosquid. I can only presume that turbosquid pass all copyright restrictions and implications in the event of copyright infringment over to the seller of the item. I guess this gives turbosquid protection from copyright infringment and im sure in extreme cases Turbosquid would also refund any monies made from the sale of such illegal materials to the purchaser and/or the owner of the actual copyright.

To be honest this is a hugely difficult area, one that even the big market movers are struggling with and its something a small start up is going to struggle to deal with, let alone rectify after the event.

I think personally you need to readdress how your going to sell materials and possibly look at internally generated materials only. Possibly invite the sellers to join some kind of development team, so that you can monitor and police the materials.

The real issue comes when you have a large volume of submitted materials to review, and to be fair, until that happens you are probably ok with a simple disclaimer passing the responsibility for such actions over to them, and ensuring that customers are aware of this before they make a purchase.

hope that helps.