I suggest Ebay. Many people now shop in eBay. I just bought a custom made wedding dress in eBay. You should check out this web site to see how people advertise in selling their dress. Must have a good an clear pictures.

http://stores.shop.ebay.co.uk/Daffodil-Bridal-Couture__W0QQ_armrsZ1

Good luck in selling you dress! :)