In eBay, your SKUs are stored in a field called "Custom Label".
To access the Custom Label field, you'll need to subscribe to eBay's Selling Manager (free version) or Selling Manager Pro (paid). Here's a link to do that: http://pages.ebay.com/sellerinformation/sellingresources/sellingmanager.html
Displaying your Custom Label field
- Once you have subscribed to Selling Manager, go back to My eBay
- On the left you will see an "Active" section under Selling Manager. Click on Active and all your active listings will be shown.
- If the Custom Label field is not already showing for each listing, then click on the Customize link.
- Check the box to add Custom Label to your view, then click Apply.