I have in the past suggested sending clothes you no longer wear to Thred-Up, but haven't used the service. While considering sending items to them I started looking at reviews and decided it wasn't such a good idea after all.
Another clothing center that a friend uses might be worth looking at for upscale clothes is Posh Mark. I haven't used this service either. Let me know if you have any suggestions on where to sell used clothes in good condition.