Once he refunded you the transaction was voided and no longer exists.
If it is still on your feedback page you can leave appropriate feedback.
However, although it sounds as if you were dealing with a dropshipper who never knows whether his supplier has any of the products he offers in stock, he did let you know promptly that he could not supply the item and refunded quickly. If he also offered a Mutual Cancellation and you agreed, then you were agreeing to the refund and end of the transaction.
At worst a simple neutral "dropshipper- product out of stock" would explain the possible problem to his future customers, without making you look like a PITA.