Excerpts:

According to the licensing agreement the Renderosity requires sellers (Artist/Author) to put on the items - 'Buyer acquires the copyright to any derivative works created using this work, provided none of the original materials can be extracted from the derivative work by any means.'
And the Buyer's portion states - 'Buyer may use the materials in any personal projects or commercial projects, as long as the Artist 's work is protected from extraction'