I am tempted to do this. There's some gorgeous Vera Wang gowns on there. Others on this board have advised me to try on some dresses locally first, to see what type of style I like. So that is what I'm planning to do, then if I see a nice one on Ebay, I can get one the same style.
You can always get them altered to fit, so I'm not too worried about the size. Average priced gowns are so expensive in the UK, and I figured I would have a better chance of reselling a designer one afterwards.
I did a Google search on how to buy a second hand Vera Wang wedding dress, and found that all Vera Wang gowns come with a certificate of authenticity. So, I'm going to avoid the ones that don't have the certificate - just incase they're not genuine. I think anyone buying something so expensive, knowing they would be selling it afterwards will keep the certificate, so that should be a safe bet. Apparently, if you take your second-hand gown into an official Vera Wang store, they can also check it for authenticity.