The buyer could give you the collection code in which case you would be able to get your payment. It is unlikely that they will do that though.
They can arrange their own courier who could bring the collection code or, if you are prepared to pack and post the item, you could cancel the sale and relist it with a postage option.
It depends a little on how much you want the sale.
You could simply say that they need to collect it in person and, if they can't, offer to cancel and refund them.
"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)
