Many thanks for all of your feedback.
In my case, I received his first email about the problem a couple days before my Europe trip, and I forgot to respond to him before I took the plane. Then, I got another email a few days after I arrived in Europe. It was about five months after I sold the amp.
I had a limited access to the Internet in Europe, and I told him that I could not help him for the problem he had -- the biasing problem -- and suggested him to try new tubes.
Then the buyer sent me an email and said how disappointed he was about my response, and went for a dispute with no further discussion.
So, apparently he seemed to have expected more help from me. Had I had more knowledge in biasing (the amp is self biasing), and had I had a better Internet connection, I might have tried to be more helpful, but as a matter of fact, I could not give any more helpful advice than changing tubes. If changing tubes did not work, then I might have advised him to contact the manufacturer.
The amp functioned just fine before I sold it, and that may be why I did not hear anything from him until five months had passed. Anyway, I felt sorry for him that the amp started acting up, but it was beyond what I could do for 20 year old used power amp five months after it was sold.
Also, I had no knowledge of how the amp was used or abused by the buyer for several months, and there was no way that I would accept the return.