You have the item and the buyer's money, so one has to be returned.
As it cannot be the item, you have 90 days to refund the buyer selecting Cancel this Order Problem with Buyer's Address. This does not then, affect your seller performance.
Click on the item, top left to view order details, far right to drop down menu More Actions >> Cancel Order.
Even though they are not replying to your messages, send a final one explaining what has happened and a refund processed which will go back to their original payment source. If paid by credit card advise them it can take up to 30 days for the refund to appear in that card account.
@wv0210