Ebay is excellent for evening wear - I have an extensive wardrobe of evening dresses from Hobbs, Whistles, Phase 8, etc etc and paid almost nothing for any of them (well usually around £10 I suppose). Vinted is also good but I find a better selection on Ebay. I know what size I am in most of these brands so my searches tend to be 'dress hobbs 14' or similar which filters out all the random chinese brand stuff. Mostly the evening wear has been worn lightly and often I get stuff still with labels on so unworn.