Like others have stated you can try CraigsList or eBay.
However, you're not likely to get what you paid for it, or maybe even near what you paid for it, unless its a very popular dress that is in demand and is hard to find. Even if you haven't worn it, it still considered a "used" dress.
Your dress may be listed on these sites below, if it is you can get a ballpark figure for the amount it would go for re-sale.
http://www.preownedweddingdresses.com/
http://sellyourweddingdress.com/
http://www.buyselldress.com/
http://www.woreitonce.com/
http://www.weddingdressmarket.com/
Here is also an article from theknot that may be useful.
http://wedding.theknot.com/.../wedding-dress.../how-can-i-sell-my-wedding-gown.aspx