It seems to me that there is some basic misunderstanding about what is meant by 'crediting' someone else's work. Perhaps I am partly to blame with my suggestion that mentioning where you got something would give the guy another sale; I didn't mean 'free advertising' , simply acknowledging what was and what was not your work. Perhaps I'm wrong but I had thought that Renderosity was an 'artists community' and as such it was for sharing methods and helping each other. In the context of the galleries (as Eric has said in his post above),  that means making it clear what you did as the artist. How else can anyone make any valuable judgement about your work?

Another analogy would be academic or scientific papers, where the work is invariable based on the work of others. Any paper submitted for publication or peer-review would be rejected without proper references. This is an extreme example, agreed, but the prinicple is the same.