What a goose, I wouldn't offer him a refund. I can't stand people who get all high and mighty and threatening. Let him open a dispute. All you have to do is reply in the dispute that it was a pick up item and he will 100% lose as there is no protection on pick up items at all, in any circumstance.

Just ignore his messages, respond in the dispute as suggested, and if he does leave you bad feedback well who cares, you don't sell for a living, so it will make no difference at all to your life. Respond to the neg professionally and politely and others will see that he is a twit as no doubt he will use capitals, spell incorrectly and say something ludicrous.

Have a good Christmas and forget him 🙂

It sounds like there is nothing wrong with the guitar at all.