Hello,
I have used Auctiva only for creating listings for ebay, both my auctions and store listings. Like previous poster ninth_wave said, your Auctiva store is not a stand alone store, it is merely a display of the items you have listed on ebay, both auction and store.
I have heard that after you create a listing and save it (don't have to post it to ebay), that you can get the HTML coding for it, and copy/paste to another auction venue (if they allow HTML) or even to a webpage of your own creation. I personally haven't tried it yet.