Ebay is the selling venue...Auctiva is just the listing service. The Auctiva store does not stand alone. If someone happens to find your Auctiva store and clicks on an item they are taken to the Ebay listing.
Auctiva gives you beautiful templates and 23 free pics, a free store, and a host of other goodies but you cannot use it without Ebay. It is used in conjunction with ebay, to enhance the listings you already have.