Yes, SoulTaker is right.

The Renderosity (and Daz and most other marketplaces) standard license says the end user can use the products for commercial work.  This is standard and this what we as the end user expect.

If a vendor is selling a mesh (and I've seen thsi on Turbosquid, 3DCafe and others) that is a blatant remake of an exsisting design (I've seen X-Wings for sale, fo example), then the Vendor is infact breaking copyright law.  The Vendor has no right to sell his version of a copyrighted design.

If you see a copyrighted design in the marketplace, you should notify the Renderosity staff immediately and that item should be removed.

I (for example) have no clue what every freaking Star Trek and Alien spaceship design looks like.  I also work in TV and I use purchased meshes and such for commercial work often.  If I purchase something that I don't know is a copyrighted work from a marketplace such as Renderosity and I get into trouble over it, I will make certain (out of spite) the the original copyright holders find their way to where that mesh was purchased.

So, to re-iterate:

Standard Marketplace license on all merchandise is that you can use it for commercial work.

If an item in the Marketplace is not an original work or a work that the Vendor has full rights to, then the Vendor is breaking the law and is endangering his/her contract with the Marketplace.

You do not have the right to use Copyrighted material for commercial work, regardless of origin (free or for sale).

There is a loophole, though, to be aware of (as the Serenity in Battlestar Galactica example shows) but you should do your research on that and be aware that if the issue is taken to court you might not win.

(I know of a comic years back that had thrown Batman into the background of one scene as a bit of a tribute.  The Comic publisher was sued, the comic was cancelled and a lot of people got into a lot of trouble.)

(Tributes and spoofs are a very grey area and you tred on shaky ground legally speaking.)