Hi Bodhiwater,
Help someone else recently with this problem even though I don't currently use ebay Store.
If you look in your Auctiva Settings in the My Account tab, you'll see a link to ebay Settings. Click on that and the page you're taken to has a link to "Update Store Categories From eBay". That will import your Store categories which then will be selectable on the Auctiva one-page lister.
Danno