Your buyer can't return the item as they clearly don't plan to collect it.
Buyers should not have to pay to collect their parcels and this is your error.
If the buyer opens a case through Ebay for 'item not received' you will have to refund them.
If you put return details on the parcel, it should eventually come back to you.
You must make sure that you put enough postage on.
"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)