Simple Delivery covers you against this sort of claim.
If the buyer had opened a return/claim stating that the item had been damaged in transit, Ebay would have refunded them.
As you have refunded, and you sent using Simple Delivery, I don't think you can claim from Royal Mail.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)
