For what my dress originally cost I'd want a fair whack back for it if it got sold 2nd hand - I'm having a boy so no chance of passing it to him in years to come thus I'm having it made into his Christening gown. That way I can keep it in a sense without having it taking up masses of space in the airing cupboard or under the beds etc. I'll never wear it again and having the Christening gown made from it means I know it started as our wedding dress but will end up (I hope) being worn by my son and fingers crossed his children and their children etc in years to come. Easier to keep a baby gown than a huge wedding gown.
Some branches of Oxfam will buy them 2nd hand from you as will a lot of bridal gown shops etc. Bear in mind though that despite it costing say £1,200 when bought new they will want to re-sell it for about £500-£700 (despite only 1 use!) and thus you'll only get £200 for it, if that. If you sell to somewhere like Oxfam they like to sell the gowns for around the £200 marker so you'll get peanuts. Worth doing some research into there are bridal stores who specialise in 2nd hand gowns, you might be better off contacting them first.
I couldn't have mine sitting around for years on end, no space for such a huge box and I have the accessories (veil, tiara etc) and the christening gown so I'm quite happy with that. I might have kept it had I of had a daughter.