KarinK, it would be difficult to tell without seeing it in person. Chanel is one of the most commonly faked items on auction sites. Unless the seller is well known for its high end resale, and willing to offer a guarantee (with full refund) that the item is verified authentic, I would run away from the listing. That said, it is still possible to purchase real items on the secondary market...but it is not without risk.
To the original poster, if you are looking for a piece of Chanel couture, I would suggest that you visit a boutique (some Neimans and Saks have them). That way you can view the real deal in person and know what to look for on the secondary market. I would also check with jewelers who deal in estate pieces and even with high end pawn shops. For Chanel fashion jewelry, I would look for very high end resale (consignment) shops. There is a wonderful one in Washington, DC in Georgetown called Ella Rue.