@DUST, the case your describing here is in violation with the terms the standard license provide, that seller is basically sub-licensing your work illegally.
He would normally need to pay you royalty’s for every model he sells (that has your work embedded in it), 3D stock media cannot be embedded in other 3D stock media unless the original creator approves to this use.
Normal use cases for 3D stock media would include using it as part in making video productions (be part of a scene) or it can be embedded in a game and sold along with it if that is in a proprietary format. In your case that seller is simply redistributing/re-selling (sub-licensing) your work embedded in regular 3D formats, that's in pure violation with the standard license. He/she is directly making money of it and competing with your offerings that get devalued in this case.
As far as I can tell you have ground to sue him and demand to get compensated for all damages he may have caused you. However, I would advice you to keep it friendly and contact that seller and point out that his uses are in violation with the user agreements and request the removal of your work embedded in those files his selling.
Also note that 3D stock media models embedded in proprietary game formats cannot be sub-licensed within those formats or frameworks, meaning, that model (design) cannot be subject to in game items provided for sale to the game players or hobbyist builders.
In short, 3D stock media cannot be sub-licensed in any way or form without explicit permissions of the rights holder.