I don't know about DAZ or Renderosity's marketplace, but I suspect most of the agreements are basically the same (My stuff is at Bbay.com). The broker gets a 50% cut of the vendor's sales (basically they provide FTP space, a central location for similarly used products, product promotion, and bookkeeping) which is not a bad deal if you don't want the hassles of selling stuff from your own site. The biggest difference between Renderosity and DAZ would be the "exclusivity agreements" where you agree to sell a particular item on their site without offering it in another marketplace.