I love ebay, but my biggest advice is to never rely on what size a piece of clothing is. Always look for items where the seller gives you measurements. Even with brand new clothes in the stores you won't always be the same size, it becomes a lot more difficult when you're dealing with clothing that's from seasons past.
If you see something that looks intriguing but they don't list measurements, email the seller. They will most likely answer you and post the response on the auction.
Happy shopping!