You should try bravobride.com, which is free. Some other good sites (some free, some not) are preownedweddingdresses.com, oncewed.com, and thegreenhangershop.com.
You can also post on our own wedding classifieds board for free.
I just sold mine on oncewed.com. It was a great site to work with and I had a lot of interest in my dress right away. I tried other sites and just never knew if anyone was looking there. Oncewed.com was a winner though!
Re: dress for sale
Miss Mrs.