It's actually against ebay's rules now to accept cash on collection. But it's only recent so easy mistake to make. Ask the buyer to use the listing to "buy" it on ebay and then you can mark it as payment received via the dropdown menu arrow for it on your orders page.
Or, as there are no fees involved you can just end it early here:
https://www.ebay.co.uk/help/ip?topicId=4146