I've bought from them before. I've never had a problem with authenticity, and it's been a while since I've seen a fake pop up, but it definitely has happened. They've grown too fast and their copywriters, merchandisers, authenticators and photographers can barely seem to keep up with the amount of product they have coming in. If you look at the size of their inventory it's no surprise, really. But they get some great bags--mint condition oldies as well as new condition current bags (as well as "fair" condition bags which they'll inexplicably price the same as or more than mint condition bags).
I know many people (myself included) have contacted them asking why they can't just take the right photographs (like
fashionphile does), but their response is usually that they have a set of 'uniform' photos they take for every bag. How that doesn't include at least the tag is beyond me.
More often what I see instead of fakes is inaccurate or incomplete descriptions, and therein lies the key to their "no return" policy. If there is anything at all about their item description that is inaccurate, including the condition being different from what is shown in the photographs, they will take the bag back and issue a refund. I ordered a white city from them once, in the photos it looked pristine and I was thrilled to find a white 06 chevre city that was like new. But when it arrived the white was actually gray, and huge patches of the bag had yellowed, including one entire side panel.The photograph had completely misrepresented the true condition of the bag, and the description just said "faint wear." I called and told them and they took it back and paid for the return shipping, no questions asked. Their copywriters are vague and/or sloppy, and their photographers often don't document the bags completely or they describe them inaccurately. Either case is grounds for a return and in my experience they'll honor it. Same goes for odors. They don't describe odors, and I've ordered two non-bal bags that stunk so badly it was like getting hit in the face when I opened the box. In both cases, they took them back and paid the shipping.
All of this said, there is always the risk that unless you have an exceedingly clear reason for the return (fake/NAD) you'll get a customer service associate who is strict and unbending, and you're stuck with a bag that you just don't like. It seems to work for them though, because they sell a ton of stuff and they keep growing.