There's nothing you can do to enforce a payment from the buyer from eBay.
Perhaps the buyer does not know how to repay you, send them another polite message apologising for the late delivery but to say you are pleased to see that tracking has shown delivery and could they kindly repay £** amount either by a cheque, bank transfer or with e PayPal Friends and Family payment.
Or, you could send them a PayPal invoice.
Was it a large amount? If so, and the buyer is not paying you'd have to pursue a legal route.
@halo33