There are some people with issues out there. Several years ago, probably about 8-10 or so. I had a similar experience on ebay. The person came unglued over a $11 beer tray I sold him. I offered to take it back, refund the money but the guy was off center. He was upset that it was not what he was looking for.
He flooded my emails with profanity, he got my phone number and made threatening calls to me and my family. This lasted for almost 8 months. I notified ebay but at that time it was not as sophisticated as today. They suspended him which set him off even more. I notified my local police and the police in his town. Nothing they could do if he did not carry out the threat.
I then did some research on the person myself. Found out he was a minister in his local community. Found out he was the leader of the ministrys chorus and a well respected member of his community. Just as I was about to notify the ministry of his behaviour the episode ended. I never heard from him again. I was prepared to drive up to his location and confront him personally to find out what was the problem. Fortunatly I never had to resort to that.
I found that ignoring him was the best course of action although it lasted for to long a time. I do suggest you check his background on the web so you know what you are dealing with. You will be surprised what you can find out about an individual on the web. Especially if he has a past history of behaviour issues. Also knowing his background and what is important to him will help if it continues. Any professional memberships he has or even family members names can be invaluable should you have to take action to stop the threats.
Good luck
Ray Homiski
Elizabeth, NJ USA