I bought my dress off tradesy but it was a new dress. I absolutely would've bought a used one had I found one there. I was familiar with dresses on tradesy & then went to the stores and tried the same ones on. It worked out great for me!
ETA- make sure you get many details from the seller especially measurements.
Also, I was able to buy a size 8 when I'm a 0. I showed a pic to seamstress before buying & she said that particular dress could be sized down that far with no problem
I found my dress on preownedweddingdresses.com (it's similar to tradesy), the lady I bought my dress from on that site actually runs another site called thelastminutebride. I was nervous purchasing online but it all worked out great.
ETA- I would def try to try the dress on at a local store before purchasing. The sites typically give measurements but it gave me peace of mind to know how my dress would fit.