As I mentioned in the removed thread, it's worthwhile to take the time to read the full
Google/Timble 3D Warehouse Terms of Service, in particular Section 11, which is about content. I researched this a while ago, as I had questions myself: the TOS can be confusing. I will summarize my interpretation (again). But, I'm not a lawyer, so everyone should judge for themselves what they are comfortable with or get legal advice.
Briefly, publishing on 3D Warehouse makes a work public, in the sense that other users may create derived works and publish them. If you don't want that to happen, don't publish there.
That said, the orginal creator retains the copyright, so claiming a derived work as your own would be a copyright infringement. In other words, it is advisable to credit the original creator. Is it mandatory? I don't know, but that way you would properly acknowledge the original creator.
So, taking a model from 3D Warehouse and converting it into an iClone model (which can be a lot of work), and then publishing it in the RL MarketPlace cannot be considered stealing. However, it would be a dubious practice to claim the model as your own. It would be decent to indicate the source of the model.
When there is no monetary gain, in other words, you provide the model as a freebie, you would simply have saved people time to do the conversion themselves.
As to whether you can charge for such a model, one position is that if you spend a lot of time converting, you could state that you charge for that time, but not for the model. Again, by indicating the source of the model, and actually providing a link, your potential customer can decide whether they find your conversion worth the price or whether they want to try do to it for themselves.
Since RL owns MarketPlace, they of course have a stake in this, and when in doubt, any particular instance should be taken up with them, as Peter (RL) already pointed out. Also, questions in general about the above are best taken up with RL.