This tutorial explains how to add unique SKU’s (Stock Keeping Unit) a unique identifier for all your products to your eBay listings.
To enable the use of SKU on your eBay account you will need to use Selling Manager or Selling Manager Pro, Selling Manager is a free upgrade to accounts, you can upgrade using the following link: https://pages.ebay.co.uk/selling_manager
It can take a few hours once requested for Selling Manager to become active on your account.
Once Seller Manager is active you need to access it from your menu, once in seller manager
Select Edit -> Edit all Items