It looks as if they asked to cancel really quickly after buying.
The sensible thing to do is cancel and refund them. If you insist on sending the phone, they will probably open a return anyway after opening it.
All the details are here.
https://www.ebay.co.uk/help/selling/getting-paid/sellers-can-cancel-order?id=4136&st=3&pos=1&query=H...
When you cancel the sale, the buyer will be automatically refunded.
"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)