Where does a sellers responsibility stop?

I was working when it was being demo'ed. He saw it working. He took it away WITHOUT letting you do what you thought was proper. (removing the tubes)
I say he is out of luck.
The problem IS his.
IF you feel like being a 'nice guy' and looking at it at his place, then go for it. But you are not responsible.
He antics and anger don't mean squat.
How do you know the rantings are not a put-on to return something he damaged? In fact the angrier he is the more likely he screwed something up himself. IMHO

I would take it back provided it still worked in your system & was in the same condition as before. If there are any signs of abuse I would refuse the refund but maybe like in Budrew's situation, the guy didn't know how to hook it up. Then again maybe it's tube related & that gets into a gray area.

If everything's OK you won't have any problem reselling if it's the LS5 because you can reverse the SOLD on the ad.

If he came to your house and demo'ed it and then he took it with him, I would say your responsibility ended at the front door. It worked fine until it was out of your sight.

It would be different if you had shipped it to him. Then it is your responsibility until he confirms it works at his house.

I think you are being more than generous in taking it back.

Those are my two cents worth anyway.

Thanks for all your responses. He brought it back and I took a close look at it and could see no sign of abuse. Audio Research is notorious for having the toggle on/off switch go out through constant use. I'm betting that it was just bad luck and it happened after he took it home. He still wanted the unit, so, we agreed that I would keep half the money until it was repaired, which I would bear the responsibility for, and then he would return to pick it up.
That seemed like a fair compromise. This way if the repairs show abuse I use the money he left to pay for it and return the rest making him responsible. If the dealer sees it as a non-abuse repair situation I pay for all the repair and still sell him the pre-amp. I think in dealing with used equipment you always should treat the other party the way you would want to be treated, even if they are obnoxious.

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...