Where does a sellers responsibility stop?


I have a ethics question that I need some input about. I just sold my pre amp to a buyer who made a local pickup. The pre has always worked flawlessly. He demoed it and then we took it out of my system. I started to remove the top to take out the tubes and pack them as they were when I purchased the unit. He said not to bother and so we packed it in all the original boxes.
He called me 3 hours later and said that the pre amp doesn't turn on. I told him that it had never had a problem like that before but, he was welcome to return it. He seemed really miffed at me like I cheated him. Now the problem is, I will be refunding his money but, how will I know that he didn't drop or short out or whatever the unit ?
Did my responsibility stop once it left the door?
agaffer
Welcome him to the land of tubes. What did he expect?
The reliability of solid state?
I brought home my Rocket 88 (with all tubes removed).
I re-installed them carefully & watched one of the EL34s turn frosty white on start-up. I replaced both tubes w/a matched set & moved on.....

This gentleman probably does not have any experience w/tubes...
Agaffer- Hats off to you for dealing with this difficult issue in a very responsible manner. I'd deal with you anytime. A creative and way cool solution.
Agaffer, the way you handled this touchy situation is admirable. I'd buy from you anytime, without hesitation.

Best regards,
Plato
Agaffer, you're a gentleman and a scholar; and there's not many of us left...you were a cut above. Nice to hear....
Agaffer- I think we are all curious as to what the outcome will be. Please post an update when the unit returns.