Store items are hosted in inventory already, so in order to turn it into an eBay auction, you just need to fill out auction information for the item and then you can launch it out of Inventory. You should see an Auction Information tab within the inventory item. There is a Site Specifics section there, to the right -- click on the edit/setup link to change information, then go ahead and save all changes. From the main inventory screen, locate your item again -- select it and click on the Launch button to launch the item to eBay.
