OP, it sounds as if you should only buy new. I say this because you don't seem to be 100% confident in having an authenticator confirm the status of a bag, so your mind may never be at ease when you buy used.
I have recently learned (via my involvement on the Chanel boards) that some people come onto our forum and try to get help in scamming (ie. to sell fake bags). I was sad to hear this, as I tend to be overly trusting, but I now understand why the purse forum cannot provide things such as a detailed library of close-up pictures.
It's a sad state of affairs, that's for sure. I only own one pre-loved bag and it is a Chanel Cambon. I am 99.9% sure it is authentic but I am not keen on buying secondhand myself. I wonder if there are some buyers who don't actually care it is not real as long as it looks almost real.