I am not an expert this answer is based on my experience.Hope it makes sense.
No, You don't have to have an ebay store to use the Auctiva store front. People are confused about what Auctiva store front is. Auctiva store front is not not a store.
Its basically window where all your ebay listings are advertised.So, if you have listings up on ebay you can use Auctiva store front.
When a person clicks on your Auctiva store they are taken to a page with all your ebay listings there.When a person clicks on an item they are taken to ebay where the item is actually listed.
I love Auctiva Store front. It has helped my sales tremendously. I am thankful Auctiva gives us a way to advertise our ebay listings making them look professional and gives us the option of a sideshow of our ebay listings as well.Click on Store to make yourself a customized look.Its great advertising and it's Free.
Now,
The reason you get an error message when you try to list a store item is because that is for an actual ebay store not your Auctiva store window.When you list on ebay you can list Auction,Fixed Price,
or a Store item only if you have an ebay store.
Someone else will have to answer your check out ?
Hope this helps.