There is always the For Sale / Wanted board on Mumsnet but you have to pay a tenner to use it and there never is anything listed. I'm tempted to cough up and try and revive it - I have stacks that I haven't got round to selling.
I believe Cloth Nappy Tree is no longer in operation, which is a shame, and I've sold a lot on there. You can sell on eBay, but you have to be careful about how you describe them, as officially you can't sell used nappies on there. I've never done it though, just browsed their listings.