Rayraz,

there is one main difference in yours and rj001 models I assume. He is selling his commercially. You are talking about someone asking your permission first. rjoo1 is for all intents and purposes not just selling his models to the public at large, but he is also selling a license for the buyer to reproduce something using his model, wether commercially or not. In the case of a cover artist for a novel where would the artist post credit for the use of this model? In the novel itself? Ask the authors permission to advertise for someones models they purchased? That's unlikely, even unheard of. If the artist had a website, or posted here at renderosity, they certainly should give credit for the use of the model, but not when it's being used in print. And I mean no disrespect at all when I say the artist/illustrator of the cover really isn't obligated to do so if they were purchased from a store. The artist purchased the right to reproduce without that obligation.

Borrowing models for free is  a different story all together though. I always get the modelers permission before I use something commercially if the modeller is doing it for free. but commercially there's just no way I can see that giving credit is even possible.