As far as selling there's always ebay, although with an overflow of discount bridal stores also selling on ebay, prices are going way down. If you have a couture bridal gown, weather it be new or gently used you can take a trip to Chicago's bridal consignment store White Chicago. Here you can drop off your bridal gown to be sold by trained bridal consultants. It stays there for 3 months and I believe you get 40% if it sells.
Donation to me is the best way to make an impact and be able to make some money come tax time. All donations are tax deductible and if you end up shipping the dress, shipping is also tax deductable. Making Memories Breast Cancer Foundation is a wonderful place to donate your new or used bridal gown. They will take all donations and go around touring in different states to sell the gowns to hopeful brides. We have made donations to Making Memories for 3 years and it is an easy process and since everyone is touched in some way by the disease, also a fulfilling donation.
There are many other ways to donate your bridal gown and make it count towards a good cause. The Bridal Garden in New York takes donations of new and used bridal gowns with the profits benefitiing the children of New York. Again a great tax deduction for you and a great way to give to others at the same time.
If you have a prom dress to donate I recommend the Sista Pride's Princess for Prom. It runs right out of Milwuakee by The Boys and Girls Club and is for a wonderful cause!
And I know you all want to hang on to those wonderful Bridesmaid dresses from the 80's and 90's but let go and make room in your closet for other things. If you think that no one would want or use a tacky dress, think again! Many Highschool Musical and Theatre departments are happy for these kinds of donations. They work perfectly for plays or musicals set in different eras, or they alter and change them to look completely different. Call your area schools to find out if they are in need, this is also tax deductable if they have a form for you to fill out.
We get calls all the time about where to sell gowns or what to do with bridal gowns when a wedding is canceled and the dress is new and unaltered. In that case it's a wonderful opportunity to take an unfortunate situation and make it into one that benefits many.