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
Actually, it's not that easy, you have to hold the counter still and turn the house.
Darlene needs to talk with our Home Despot specialist, Johnnantais. His Lenco torque wrench would tighten that counter right up.
Another great post and a fine example of what this community is all
about...way to go 'agaffer'. 
This is a hard question for me. I remember I sold a pair of headphones and a year later the buyer is complaining to me that I modified the headphones and screwed stuff up in the process. I told him I did no such modifications and just plugged them in and played them which was the truth. I gently reminded him that it had been a year and if he had a problem why wasn't I notified a lot earlier. I would have taken action then.

Now whenever I sell I do a full test before I ship the item out if it has been sitting for awhile. I'll also state up front that this is an as/is sale and No Buying/Trying/and Returning if the gear doesn't have synergy with your system.