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
My Ebay > Activity > Selling Manager Pro
You then need to go into Inventory and then click on Customize
From Links available select Custom Label and make that Links Selected to Display
Now just click on Custom Label on Listings to add your SKU to your eBay listings.