If it was listed with postage you can mark as dispatched by opening the postage label. But you won't have proof of delivery. It would be safer to cancel the transaction (with buyer's agreement) via the dropdown menu arrow alongside the item on your orders page. Then relist as collection only (your delivery will be treated the same way) for the buyer to buy and pay again. They will receive a code to give you when you drop it off. See "Proof of collection" on here: https://www.ebay.co.uk/help/selling/posting-items/setting-postage-options/local-collection?id=4181
