There is a thread that discusses "Trusted Chanel Sellers" somewhere around here. A lot of ebay discussion is on it though.
My rule of thumb--be trusting, but still vigilant for quality and errors, when purchasing from Chanel / Barneys / Bergdorfs / Neimans etc. that sell current season Chanel, new in box. Always 3rd party authenticate and assume the worst when buying from any reseller be it full fledged companies like Fashionphile or personal sellers on ebay. I'm wary of the mistakes and 'benefits of doubt' that come into play when the same group/person is authenticating and selling the bag. Would rather get the bag checked out by a 3rd party authenticator that does not profit by letting false negatives slip though.
I've been looking into how to tell fake bags from real and some are definitely more obvious than others.I know Yoogis closet has a good reputation and say they authenticate all bags, but one on their site actually looked fake to me but I don't know enough about Chanel to say for a fact that it was or not.