For the eBay Integration for Zen Cart Plug-in to function properly, the "Out Of Stock" preference in eBay should be enabled. To enable the preference, log into eBay.
Select Account Settings from the drop-down menu under your user name in the upper left corner of the eBay web page. Under Selling click the Selling Preferences link.
Look for Multi-quantity listings and turn on the Listings stay active when you're out of stock option.