By default, your listings end when your items go out of stock. If you don't want your eBay Good 'Til Cancelled listings to end when the quantity goes to 0, you should configure eBay's out of stock control setting in your eBay account. Here are instructions on how to configure this:
- Go to My eBay->Account->Site Preferences.
- Under "Selling Preferences," click "Show" next to Sell Your Item form and listings.
- Click "Edit" next to "Use the out-of-stock option."
- Select the checkbox, then click "Apply."
Please note that if your item remains out of stock the entire 30-day billing period over 3 consecutive 30-day billing periods, eBay will end the listing and there's no workaround for this.
Please see here for more information about this feature: http://announcements.ebay.com/2014/12/sellers-new-out-of-stock-option-helps-protect-your-hard-earned-seller-standing/