As far as I can tell you cannot sell slightly modified conversions, if you want to avoid problems then it’s best you contact the original author and ask him/her.
Its not allowed to sub-license a 3D stock model, the royalty free license has a paragraph explaining some exceptions for models to be part of a game in a proprietary format, but as you present it you bring the model to market not specifically as part of a game but just the model itself (albeit in proprietary format).
Basically you covert the model to be compatible with some game world and then sell it to users of that game world? That sounds like in game items for sale internally or externally and 3D stock cannot be used for that as it equals sub-licensing.
Personally I would not approve my designs would be used that way.
I would be fine with someone contacting me and telling he potentially has a new market for my designs after he has applied his expertise on modifications and conversions making them compatible for some game world. However, we would need to come to agreements on the proper amount of royalty’s per copy made/sold. He would also need to find a way to connect me with the market he is selling to so I can view how many copy’s are being made.
The point is he will not become owner of that design after some modifications and conversions, copyright will remain in place until agreements are made for maybe special license, for example, exclusive fee based on estimated amount of copy’s involved could also be subject for negotiations.
I’m just providing how I would deal with my copyrights to give you some perspective but keep in mind someone ells may have different ways of dealing with it so best contact the artist and negotiate with him and ask for written and signed permission to avoid problems with that specific type of uses.
