Reselling any formal dress is hard, I sold 3 ball gowns on eBay for the princely sum of £90 each last year, each was over £500 new and only worn once. It was quite upsetting, but I needed the money to keep the roof over our heads. Have you tried approaching a dress hire agency? Did you buy it from a shop, any chance they'd take it back? It's so sad to have to sell your prefect dress, I hope you can get a good price.
I am in a similar position and have to sell my wedding dress as we've cancelled our wedding due to my dad dying unexpectedly. It cost £1250 and we have no plans to marry any time soon. I have also tried e bay no joy there, have you tried gumtree? Sometimes face book has wedding selling pages for your local area? I have tried ringing several shops but none were interested. I feel i may have to be prepared to lose alot of money on it, i'm currently waiting for insurance claim to go through with fingers crossed. Good luck selling your dress!
I'd say try ebay & preloved websites- however, it may take a v long time (or never!) to sell.
Currently looking for a 2nd hand wedding dress for my own wedding next summer...judging by the dresses I've seen on ebay, sellers are looking for a minimum price rather than auction.
This results in the starting price being v high, they don't sell & simply re-list every month or so at the same price. In your situation it sounds like you need the money ASAP...I would agree with the other poster who suggested going back to the original retailer.
Unless you get v lucky & find somebody who wants the exact same dress & similar and prepared to pay several hundred pounds for what may be viewed as technically a 2nd hand dress...you may have to take the hit on price.
However, your dress has not been worn so it's quite different to a lot of the ones on ebay which have some damage, staining etc. Good luck