No, Ebay are not responsible.
Did the buyer get the item less than 30 days ago?
Have they opened a return through Ebay or just messaged you?
If the buyer opens a case and says the item is faulty, you will have to provide a return label and refund the buyer when they return the item.
"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)