Does the postal tracking record confirm that delivery was attempted?
If so, eBay's money back guarantee policy states that they will treat attempted delivery as having been successfully delivered. This means that eBay will not hold you liable to refund the buyer if they open a case under eBay's money back guarantee.
However, be aware that if they paid by PayPal they can still file and win a dispute, as PayPal requires evidence that the item was actually delivered, not just that delivery was attempted. I suspect that a credit or debit card issuer would do the same if they issue a chargeback.
In your position I'd probably wait until you either hear from the buyer or receive your item back from Royal Mail before doing anything.