Nick,
I'm not aware of any particular site for this purpose. However any of the keyboard shops would have secondhand keyboards that they have taken as trade ins and would be pleased to quit. I'm sure that if you are a genuine buyer but with no time pressure to buy you could get a reasonable price.
In Melbourne I would suggest Keyboard Corner at Boronia, I've bought many a new keyboard there and they have always been fair people to deal with.
Keith