The SKU in eBay is called 'Custom Label'.
You can either bulk update the custom label for all your listing or manually update using below methods.
Bulk Update via a File Upload
The first step in updating your custom label field in eBay is to download an eBay file of your active listings. Please navigate to your eBay account -> click ‘My eBay’ -> scroll down and you will see a ‘Seller Tools’ section on the left hand side of your screen -> Click ‘File Management Center’
Once there, to download this file, Click ' Create Download Request' on the right hand side of your screen under the section ‘Downloads’
Once you have the file from eBay, please copy the correct SKU into the eBay custom label column on your eBay file.
The last step is to upload the eBay file back into the eBay exchange site by clicking ‘upload file'. This will set your new SKU# as the custom label in eBay!
Once this is completed, please logging to your Selro account and remove the eBay items from Selro and re-download.
Manual Update for Each Item
Please make sure that you have subscribed to Seller Manager. Once you have subscribe to Seller Manage, you’ll notice that the selling section on your eBay account has changed a little. Under 'Selling Manger' on the left-hand bar click on 'Active'. Once your Active Listings have come up click 'Customise' on the top right.
Once the 'Customise' popup appears tick the 'Custom label' box and apply. Custom labels will now be displayed beside your listings.