Manage Stock > No, Backorders

If you use Backorders or do not Manage Stock, additional settings are required. Please read more here:
for versions based on the Magento v 1.x http://docs.m2epro.com/x/CwAJAQ
for versions based on the Magento v 2.x http://docs.m2epro.com/x/yAEtAQ

Note:

If Manage Stock is set to "No", item quantity set in Quantity to Be Listed option will be displayed on eBay.

If Backorders option is enabled and actual item quantity is more than the one set in  Quantity to Be Listed option, the actual product quantity will be displayed on eBay.

If Backorders option is enabled and actual item quantity is less than the one set in Quantity to Be Listed option (e.g., X), the X quantity will be displayed on eBay.