If the buyer is claiming that the item was not delivered, then you have to reply and give the tracking number showing that it was delivered.
It is possible that someone else in the house put it aside carefully and forgot to tell him. Or that the package was stolen from his porch.
None of which are your problem.
You seem to have Proof of Delivery.
Tell eBay what it is.
If the buyer is claiming that the item is Not As Described, you can , again, in the Dispute, tell him to return it for a refund.
Don't argue about this. He may be right.
If the item he bought was red and he expected cherry but got scarlet, that is a Buyer Remorse case and he has to pay return shipping.
If he doesn't prove that you have it back, he loses the case and you keep the payment.
If the item be bought was red and the one he recieved was blue, that is Not As Described, your error, and you have to send him a Return Shipping Label.*
If he doesn't use it, you will not be charged for it, he loses the case and you keep the payment.
If he shows that you have it back, you refund the original payment promptly.
If you don't, eBay will.
Then they come after you for their money (and they have your financial info, remember) and you also get a Defect on your selling account.
Not every transaction will go perfectly.
Sometimes the customer is right.
So put on your big girl panties and deal with it.
*It's more complicated if he is in another country.
