Your listings must be set to GTC (Good Til Canceled) and also be set for out-of-stock. This is the only way you can keep a listing alive with 0 stock, so that PriceYak can bump the quantity up when the item comes back in stock.
Even if your listings are GTC, without the out-of-stock option selected, your listings will end when they reach 0 quantity. Here's more from eBay:
Good 'Til Cancelled listings and the out-of-stock option
If you're selling multiple identical items in a single fixed price listing with Good 'Til Cancelled as the listing duration, you can select the out-of-stock option in My eBay > Site Preferences to help manage the visibility of your listing.
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, but is hidden from search results. 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.
Using the out-of-stock option on your Good 'Til Cancelled listings may help protect you from receiving a transaction defect for running out of stock. Learn more about our seller performance standards.