Buying a dress on Stillwhite?

I love the idea of saving money and also being sustainable by buying another bride's old dress on Stillwhite. But also, I know that I can't return it once purchased, and I don't feel confident enough that I will love a certain style on me without trying it on first (let alone knowing if it's the right size).

Has anyone found a way around this? I'm thinking if it's a style from a designer that's still for sale, I could order the same sample size online from the actual brand, see if I like the style on me, and then if so, buy it from the former bride on Stillwhite.

A lot of the dresses there are cheaper than the new versions, but still not cheap enough to throw that money away if I don't like them.