They are like a online mall for consignment shops and places like that to also have a online store. So basically you can find a quite a few different stores on the site. I have never purchased from them though. Hope that helps a bit.
All vendors who sell thru Malleries are pre-screened and held to very high standards. All items offered must be authentic and accurately represented. They have a rigid zero tolerance policy for counterfeits. Additionally all are professional business people who are experienced in all aspects of customer service.
That being said though, it is still recommended that you run any item you are interested in purchasing, by the authenticators here for further peace of mind.