First off what did the buyer think was wrong and were they justified in expecting a full refund?

I have never had a buyer threaten anything as I have always managed to sort things out to our mutual satisfaction. In the main that means if I have made an erroe, even a very small one, the buyer will always be offered a full refund with me paying for the return if I want it back.

IMO anything less is just poor customer service and I would expect to receive less than pos feedback.