Is it allowed to sell rendered still images from 3D models?

No and no. Rendered preview images are not the part of the product and seller owns all rights to them. You can't use them commercially, unless you obtain explicit permission from the owner. This was discussed many times in the forum, you can use search and read more indepth explanation.

As far as I know with “incorporate” they mean “sufficiently” altered in a new scene or other composition, not just take a copyrighted design render it unaltered as is and call it a new product just by rendering a 2D image of it.

The license grants you the right to use the work in a new work, rendering the work to a 2D image is not a new work.

New work means original work is almost rendered unrecognizable in the new work, it can be part in there somewhere but not be the work on its own.

Hi there,
Thanks for reaching out.

If the model is under Royalty-Free license, you can use it as long as it is incorporated into the product and as long as the 3rd party cannot retrieve it on its own in both digital and physical form.

You cannot resell the model you bought in its digital form and you cannot resell it in its printed form as a separate, single item. Let's say you buy a printable bust of a celebrity, print it and sell it - it would be considered as a copyright infringement. But if you'd buy a figurine for a chess game and sell the whole game together, it'd be fine.

We also always suggest buyers discuss the usage with the seller and gain their approval, this would make you a hundred percent sure that you can proceed with your decision of using the purchase.

Kindly,
Agota from CGTrader

@Agota, would CGtrader consider to make some changes to the license agreement or add some Q&A section/link? This type of question has passed several times on this forum now.

In its current form the license protects the 3D model in its digital form, but it is less clear about restricted uses of the design a model is depicting.

For example, can we just render the output of a 3D file (maybe just change some camera angle) render a 2D image of it and call it a new incorporated product (sell the image as 2D stock), the license is not clear about that yes?

I think the license should mention both the 3D model as well as the design it is depicting are copyrighted works and have limited commercial uses. It should be clear that unaltered designs, scenes, 3D files (just click and render them) are not sufficient to call it a new work/product.

I have made a custom license and consider to apply it to all my models so very mush appreciate an answer to the questions.

And we have a real example with a real case. Customer have simply rendered our models, majority on a flat color background. They were definitely not used as a part of media project. We are of course not ok and reported this, resulting in videos removed from shutterstock.
Owner has came to us and we are in a process of resolving the situation.
However we have received an ultimative statement from CGTrader that customer have the rights to sell model on stock in any form. More interestingly it ends with

"This is not a case that allows room for disputes - if you want to control future use of the items, upload them under a custom license instead."

No plans to dispute the case with them as requested, so we stopped all publishing and are updating 650 products to custom license, as of course we don't allow reselling of simple renders of our models (70-100% of composition our work) on any stock.