Personally I'd pass. I still don't see how the blue could get so washed out in the close-up pics and still have the pen marks visible. But I'm really gun-shy about buying Coach on Ebay anyways due to the massive numbers of fakes on there.
You could ask her for more pics, maybe in the sunlight? The bag looks totally authentic to me.. Ebay can be scary but there are a lot of great sellers/buyers on there too.. I happen to be both!