You did have a choice. As a private seller you don't have to accept change of mind cancellations or returns, only business sellers do.
If you have not already accepted it, wait for four days and if the buyer hasn't paid you can cancel it as unpaid, see: https://www.ebay.co.uk/help/policies/payment-policies/unpaid-item-policy?id=4271
This is the best and safest way to cancel a sale, where applicable. Neither party can leave feedback; any already left can be removed. The buyer will receive an unpaid item strike for not paying, which will stay on their account for a year. Two strikes mean that most sellers will automatically block them from bidding.
If they have already paid, you could refuse the cancellation request - but this runs the risk that the buyer will wait for it to arrive, and then find some reason why it is "not as described". They could open a money back guarantee case which you would have to accept, and also send them a prepaid return label. They may also leave you negative feedback. eBay almost always accepts the buyer's word for faults.