Vintage Five is a seller from Japan but they have an easy to navigate website, free shipping and also a boutique on Malleries. They also have this great page to help you distinguish between the fakes and the real thing. (My caveat: I have not ordered from here personally but just did some research on the web and they appeared to have good reviews and they do take returns)
Yoox.com is a great place to buy off season designer items and they also have a small Chanel jewelry section. And lastly, I got confirmation from the girls at the Purse Forum that Vineyard Vintage sells authentic pieces and they have quite an extensive selection so definitely check them out if you have a particular piece you are looking for. They also have a great page to help with authenticating pieces. Great to bookmark on your phone if you ever come across a vintage piece while antique shopping and you're not sure of the authenticity.