You felt their wasn’t an issue and the buyer thought there was. That’s a difference of opinion that should have simply been resolved. I respectfully think that you were wrong to refuse the refund, and that it wouldn’t have made it this far otherwise. The purchasing party has already admitted to being pickier than most and is obviously a collector. Why risk a blemish on your feedback over something so trivial? As far as I’m concerned.... your clearly off the hook due to how things played out, but again.... it wouldn’t have come to this unless you refused the refund that was seemingly asked for shortly after delivery.
Fair enough. Thinking back, that probably would have been the best thing to do. Sometimes life and "feeling you are in the right/ego" get in the way.