Hi Lost World Books,
Thanks for contributing to our forum. Although our Inventory system would technically allow you to assign an internal “Custom ID” to each product and could be used for this purpose to some extent, the work that would be required to get all your products into the Inventory system might not be worth what you would get from it in this regard.
Our Inventory system is primarily designed to allow sellers who order (and reorder) their products in bulk keep track of how many each product they have on hand and, if desired, set up automatic relisting and notifications based on that information - and each product must be entered into the Inventory system using the “Add Item” form individually.
If you are selling unique products, adding each one to the Inventory system before listing them to eBay would definitely increase the overall amount of time it takes to get your items posted.
However, we did add an “Add Note” option to the new versions of the Listings pages that can be used to add personal notes to items that would not be visible to buyers, you could add a note (like your inventory number) to each item on the Active Listings page and they should be carried over to the Closed Listings page when your items end.
You can access the new version of the Active Listings page by clicking the “Try out our new and improved Active Listings page!” link in the upper right portion of the default Active Listings page. There is also a similar link on the default Closed Listings page that will take you to the new version of the Closed Listings page.
Perhaps some other members of our Community who have direct experience solving this particular dilemma will come along and share some ideas as well.