Selling wedding dress

I don't know where's best, but people are selling a lot of dresses on Vinted. also maybe look at second hand wedding fairs, my friend organises these and they always attract a lot of attention from people who have a tighter budget.

I suppose it depends entirely on how much you want to sell it for though, you won't get anywhere near purchase price for it if it's second hand, and especially if it's been altered to fit you rather than it being off the rack so to speak.