Yes, you have to be an eBay Store subscriber to list Store Inventory items.
All listings from Auctiva don't show in the Store until after they post on eBay (and sometimes have some delay until they do). I believe it is in part due to needing the listing number and some initial details that sync back for the Store. Auctiva also uses the gallery or header image for the listing to use as the Store and Scrolling Gallery image, and I've noted the image is stored at Auctiva using the item's listing number on eBay. Auctiva keeps fairly tight controls on what appears on the Store page, and I believe that's in keeping with their 3rd Party Cerification with eBay.
Danno
"Life is pain. Anyone who says differently isn't paying eBay and PayPal Seller fees."