There were some changes recently regarding non payers and feedback.

As soon as an Unpaid Item Dispute is opened the ability for the buyer or seller to leave feedback is blocked by eBay.

If the buyer ends up not paying and the seller closes the UPI dispute then neither the buyer or the seller can leave feedback.

This message from eBay comes up if as a seller you try.

Sorry, it's not possible to leave feedback for this transaction.
Feedback has been disabled because payment was not recorded

If the buyer ends up paying and it is recorded by eBay then it is no longer an Unpaid Item so normal feedback rules apply.