Paintings, images and book covers are a definite no go but regarding most of the other real world items I think we somehow collectively agree most brands not see damages done to own real products or its sales thereof due to the existence of these digital copy's. They would probably make demands if a digital copy gets sold to masses in some digital world like a game but 3D stock seems to be different in that regard, but technically its copyright infringement.
Probably general consensus is, if the activity around the digital copy in some way supports the real world product and its popularization thereof, then its ok. If it undermines or influences the brand negatively or the sales of the digital copy is significantly large, then its obviously not ok.
This is not written somewhere but it seem most take this approach to it, probably safe to say its a gray area. Anyways, discussions around digital copyright are ongoing, for example EU has recently approved the “EU copyright directive”, it will have massive impact on how Youtube and social media is used, I would not be surprised new rules like this will eventually impact this market as well. If this trend continues then its not unthinkable in near future CGtrader probably needs to connect to some AI service that looks at all the contend uploaded and automatically filters copyrighted materials.