If you paid for a label via ebay you can cancel that. Go to orders, select Postage Labels on the left-hand side, then go to the actions dropdown menu next to the label and select Void Label.
Then see if there's an Edit Tracking link for the item on your orders page. If not you can contact ebay and ask them to change it, using callback on here:
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
When you buy your own postage remember to add the tracking number and mark dispatched manually via the dropdown actions arrow for the item on your orders page.
