If they're willing to sell at that price then no reason to feel bad. Unless you were holding a gun to their head, they could always say no and hold out for more. It's only worth what someone is prepared to pay for it.
Don't worry, hang wringing lasted all of 30 seconds. To be honest I probably paid over the odds anyway but YOLO. All the compact framed sensible looking tandems can stay on ebay, with a bit of TLC and some careful component choices this'll be a banger.
It was a serious low-ball, felt a bit bad, but I guess he'll be happy to get rid of it and we'll be happy to ride it.