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?