The quantity must be a valid number greater than 0

By default, eBay supports listing products only with a positive quantity. If you receive this error, increase the Shopify quantity and send the item again.

If you would like to list items with 0 qty, consider changing the settings on your eBay account to use Out-of-stock control. Check the dedicated guide for more details on using this mode: How to send 0 stock to eBay

After updating the settings for Out-of-stock control, send the item again and check the results.