I've been stalking CC pieces on ebay for about a year. Most of the time the pieces are authentic from what I can tell. It wasn't until a week or two ago that I saw some that were suspicious and the price was super cheap. I just checked and I don't see them there any longer.
if it is a necklace like the one on ebay you posted a link to, you will probably be able to tell by holding/examining the jewelry. the cc skye bag i bought has the leather/chain bracelets on it (they double as tops to the bag), and i can tell a fake one by holding/examining the leather/chain.