I agree that you have no obligation to take the unit back, but if I was in your situation I would. In fact, anybody who wants to return something I sell them based on being disappointed, and they do so promptly, I would undo the deal assuming shipping both ways was on their nickel. If it was my mistake, I'd pay shipping both ways.
I sold a HT receiver once to a guy and he couldn't get one of the inputs to work. It was only because it was configured differently, but I didn't have the manual anymore and trying to coach him wasn't working. I think he just wanted to back out and this "malfunction" was his reasoning. He was a good guy and I don't think this was a normal mode of operation. In any case, we undid the deal on his nickel and I sold to somebody else. Somewhat a pain, but it all worked out better in the end I suppose.
I sold a HT receiver once to a guy and he couldn't get one of the inputs to work. It was only because it was configured differently, but I didn't have the manual anymore and trying to coach him wasn't working. I think he just wanted to back out and this "malfunction" was his reasoning. He was a good guy and I don't think this was a normal mode of operation. In any case, we undid the deal on his nickel and I sold to somebody else. Somewhat a pain, but it all worked out better in the end I suppose.