eBay’s enormous growth has spawned an ecosystem of third-party providers catering to auction power sellers. Services such as Andale, Marketworks, Vendio and Zoovy offer storefronts that improve on the functionality of eBay Stores, allowing sellers to benefit from Bay’s marketplace and traffic while building awareness of their own brands. But many of these third-party providers also equip merchants to sell through competing services such as Overstock.com, Amazon or Froogle (Google’s shopping portal), which could cut into eBay’s fees.

The stakes are significant, as evidenced by the growth of the Professional eBay Sellers Alliance, whose 700 members generate more than 70 million transactions and $1 billion in sales annually for eBay. With Kurant, eBay hopes to retain control of the backend, offering a way for power sellers to create an their own online identity without having to port their product database to another provider or storefront management system. For some power sellers, the move to diversify is at least partially driven by grievances with eBay, especially over increases in fees.

ProStores offers eBay a competitive service that may keep some of that revenue in-house. eBay is not hosting the service itself, however, but running ProStores as a private-label service managed by Florida-based web hoster Affinity Internet.