Feedback is voluntary and is not used by eBay to assess seller accounts.
an item that was refunded that has never shown up.
I'm assuming that you opened a Dispute and the seller refunded promptly?
If you did that, but eBay refunded you rather than the seller himself (and it is not easy for the buyer to tell the difference) your seller got a Defect.
He may think getting positive feedback will have some effect on that.
It won't.
Any feedback left for a buyer must be positive.