Boy, what a bummer. It could be something simple. Did you walk through any trouble shooting with the buyer? If you offered a refund he shouldn't be miffed; a refund is generous. You're not a dealer where follow up service is expected. In the used market it's buyer beware. The problem is, the buyer could have done anything with the preamp once he left your house. You have no idea way to know what happened. After you refund his money your stuck with a broken preamp, so i guess it's seller beware too.
I once had a guy return a CD player because it wouldn't play through the digital out. When he returned it I plugged it in and it worked great. Turns out he was using the digital out direct into a home theatre preamp that didn't have a DAC. He just didn't know what he was doing. Turned out OK because I still have that CDP and love it.
I once had a guy return a CD player because it wouldn't play through the digital out. When he returned it I plugged it in and it worked great. Turns out he was using the digital out direct into a home theatre preamp that didn't have a DAC. He just didn't know what he was doing. Turned out OK because I still have that CDP and love it.