There are a few ways this is occurring.
1. The toggle option to "keep sold items active" randomly reverts to the "on" option. You obviously wouldn't notice unless you're paying close attention. When you end and sell similar, the sold item now reverts to inventory of "1" and is available for purchase.
You can find this option at seller hub>account settings>seller preferences>keep sold items active toggle option
2. This is the most common cause: When you bulk end a group of items, occasionally there's an error, with 1 or sometimes multiple listings. If you notice it, you'll see a red outlined Ebay message indicating there was a problem ending the item. Again, unless you're paying close attention or have business policies active, you won't notice. Here's why business policies comes into play.
Ebay will packet the items with an error into a business policy, but, when you go to look at the items in the policy, there's none. What you have to do is modify the policy which will trigger an Ebay report. Open the report in Xcel, and you'll see the associated item number(s). From here, you can salvage the listings, by selling similar.
But, here's the most important piece of information. The items, although ended, will become "unsold" again on at the end of the 30 day cycle for each particular item. If you've followed the above steps, you simply need to delete the item(s) once they become "unsold".
I know this is convuluted, but, I have extensive experience with these processes. If you want to save yourself from "out of stock" defect, you"ll take heed.
This is my experience with the so called "ghost listings".
