I think that's a great idea, The problem with Charity shop dresses is they are often hit and miss, if you sell pre-loved dresses and you ensure the quality is there you could have a great market.

Vintage wedding dresses often cost a bomb but there often one of a kind.

You could start online and test the interest. I would be very interested

Think it's a really nice idea (& there is a shop in my area that sells only pre loved wedding dresses and does alterations) and the idea that part of the profit (since there is still running costs) is really nice. The bride that Donets the dress could say where she would like the money from dress to go nice touch.