Some are the bags are from Bluefly - but they list other sellers as well and all are guaranteed.
I just did a search, and there are several threads about buying bags from Amazon. Looks like it's hit or miss if you buy from Amazon directly (not a third party seller). So you should know the brand before buying, and if it is a fake, you can return it.
best idea is to post specific bags in question in the appropriate Authenticate This! sticky.