Once you refunded, the transaction was complete. As far as eBay is concerned the options they give for cancellation are the options you can choose from. There is one for 'I stuffed up', which would seem to be appropriate in this instance. It might get your fees back, but I suspect would also generate a critical defect. Maybe wear the loss of fees as a lesson for the future.
You possibly should have communicated with your buyer. This way there is a fair chance of a deserved red dot.
