I used to have a lot of luck with Freecycle -- about 1 in 10 either wouldn't go at all, or the people wouldn't show up, but I got rid of so much stuff that would otherwise be unmovable. And then it moved to Facebook, and I just gave up (everyone puts photos on things, and argh, that is too much commitment).

I suspect there would be any number of people interested in the lap harp, although whether they would be willing to pay an appropriate amount of money might be a different thing. There may well be appropriate groups around where you can advertise it -- around here, I would try the Irish Club :)