If the stuff were authentic they wouldn't need to state in the product descriptions that the bags have "all the correct markings" and that the dust bag is the "correct color". Authentic retailers don't say things like that. Suppliers of 'copies' need to make those claims.
BTW, I stick with eluxury.com, ashford.com and netaporter.com. Those I know are legit. I also have had success with jemznjewels.com. Some of their stuff is new and some is used but the woman who runs the business (Rachel White) is 1,000% upstanding and trustworthy.
I've pursed bags at efashionstore.com (fendi, lulu guinness, kate spade) and everything has been authentic. They have 5 different sections (including CA designers and yes, a "faux" section, but all separate and distinct). Also, they also are wonderful as far as customer service.
I think there prices are low on many bags because they are not necessarily from the current season's collection.
When I'm not sure about a site, I always read up on the information the site provides about the company.
If they are not willing to state their street address, phone number (other than an 800 number), I would be suspect. Also, always check out the Better Business Bureau to see if the comapny has any complaints.