Hi,
Yes, that does seem to be a limiting factor. It is odd they don't have an "Always Relist" as one of the checkable boxes on the right.
I don't see those boxes on left covering both a sale and non-sale situation for an always relist condition and hoped the "Don't Auto-Relist based on items sold" was an always method. Perhaps Auctiva wants to safe-guard you from runaway relist by making sure you have inventory (which is eBay policy).
If you do check the "At least X items are available in Inventory", that method seems to be workable. Relist and Inventory is working for a few that have posted success with that combined method. I know that's a little more work, but it does present a method.
Danno