Did you supply the buyer with a return label?
Have you checked that you used the correct address? There are 5 different places for your address on Ebay.
https://accountsettings.ebay.co.uk/uas/addresses
"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)