Royalty Free License permits use in stock photography. If the author sells his 3D models under this license, then he also gives permission for this and no separate permission is needed.
"General Terms and Conditions".
21.2. The Buyer’s license to Product in this paragraph is strictly limited to Incorporated Product. Any use or republication, including sale or distribution of Product that is not Incorporated Product is strictly prohibited. For illustration, approved distribution or use of Product as Incorporated Product includes, but is not limited to:
- as rendered still images or moving images; resold as part of a feature film, broadcast, or stock photography;