I think it's quite hard to sell bridesmaid or wedding dresses for any meaningful amount. I see a lot of people say they are going to sell their wedding dress to justify going over budget, but you see the same ones languishing on eBay for months or years on end.
I bought a popular design of bridesmaid dress for my girls - they are £70-80 each on the Little Mistress website and I paid £15 each for them on eBay. Same for my flower girls dresses, they are about £50 from Monsoon and I paid around a tenner each for them on eBay. They are all in perfect condition and the girls can keep them, but at least if I wanted to sell them I'd get my money back