When listing or delisting products to ebay, very rarely, you might encounter this error:
The quantity must be a valid number greater than 0.
This error usually indicates that a setting in eBay must be changed in order for the stock of the product to update to 0.
To solve the error, check if the 'Out of Stock' feature is active on eBay for your eBay profile. To do this and to activate the feature:
Navigate to your eBay Account view
Click on Site Preferences on the left
In the Selling Preferences section, click the checkbox to enable the 'Use the out-of-stock-option'