You calmly tell him that he should return the item for a full refund.
When it arrives (probably with Delivery Confirmation) you refund his original payment.
Then you relist the item.
And kick the cat. I keep a stuffed cat in my office for just this sort of situation.
If you get all pi$$y with him, you will get a Paypal dispute, be forced to refund, recieve negative feedback and trashed Detailed Seller Ratings, all of which will make it harder to list and sell on eBay in future.
You can't leave negative feedback. You can leave a Response to feedback, yours or his. The most effective Responses are calm and factual, but they are not required to be positive.