I don't think this is within the control of Renderosity. There are so many similar sites now, selling tens of thousands of similar products, that the market is oversaturated. It's more of an expression of supply and demand - too many products competing with each other for a stagnant market. If it's true that Renderosity has 7,000 products for sale, most of which don't sell, then the law of supply and demand will take effect. Renderosity - and similar sites - will have to cut corners to carry all the overhead items, or they may be forced into really Draconian cutbacks. So they apparently decided that one corner they would cut is to decrease the merchant's cut from 60% to 50% if the merchant's volume was low on exclusive items, possibly in an attempt to recoup costs of carrying non-selling overhead items.

These cuts can never be pleasant or seem fair to those they affect, which is how capitalism works. A communal system would make everyone happier, but communal systems don't survive in competition with capitalist systems. Merchants may get hurt in the process (leaving Renderosity and losing sales), and some may disappear (leaving Renderosity and finding there is no other market for their items).