When you enable the out-of-stock option:
All of your existing and future Good 'Til Cancelled listings use the out-of-stock option until you disable the option.
When the quantity of the listing becomes 0, your listing stays active on eBay, but is hidden from search results and customers are not able to make a purchase.
The quantity in the listing can become 0 either through buyer purchases or you can revise the listing and set the quantity to 0.
When you increase the quantity of items available, the listing is eligible again to appear in search results.
If you change the duration of your listing to anything other than Good 'Til Cancelled, the listing is no longer out-of-stock enabled and ends when the quantity becomes 0.
You can still end your Good 'Til Cancelled listing yourself at any time.
To enable the out-of-stock option:
Go to My eBay > Account > Site Preferences or Seller Hub > Overview > Shortcuts > Site Preferences.
In the Selling Preferences section, click Show next to Sell Your Item form and listings.
Click edit next to Use the out-of-stock option.
Make your selection and click Submit.
For additional details on how the out of stock option works, visit http://pages.ebay.com/help/sell/multiple.html#outofstock