Once they use the "refund" feature on paypal there's no way to leave feedback because the transaction is marked as incomplete.
OK... apparently in September 2015, there are some changes to the process.
Refund because they refuse to ship will count as a "Defect" on the seller's record, unless they Actually Lied and claimed that the buyer requested to cancel as the reason chosen for refunding. If they did that, I would definitely contact eBay customer service and complain about the seller and them cancelling the transaction without permission.
I would try the old Seller Non-Performance complaint form to eBay customer service, anyways, though.
I wonder if there is still a workflow that allows still leaving negative feedback, such as finding a "Mark transaction as payment sent" option somewhere, and then post the Negative FB.
It doesn't help that eBay keeps monkeying with their website design lately.