Sadly this likely won't help you but in Brazil they've got one site, called Estante Virtual [1], for a network of almost 1,500 independent used bookstores across the country. Even though I no longer live there, I use them as much as possible (ordering books to friends homes if they're going home to visit). While the site charges these indy stores between $12-40 USD per month for the chance to sell on their site, I would think it'd still be a good deal for the stores.

I'm not sure if this kind of used bookstore network exists in the English-speaking world but I'd love to know if it does.

1 - http://www.estantevirtual.com.br/