eBay have banned cash on collection. Buyer is supposed to pay online through ebay and then give you a QR code to confirm collection.
I don't think the option to mark as dispatched or paid for is there these days but if it is it will be via the dropdown menu arrow for the item. If not you could contact ebay and see what they can do. You can use callback on here:
https://www.ebay.co.uk/help/eua?id=5275&mkevt=1&mkpid
If they won't help you'll just have to leave it.