I'm not an expert on any of the brands being sold, but just from browsing the website I would bet that it sells fake goods. From their policies page:
BrandNameJeans.com disclaims any indication that it is an authorized dealer or agent of any of the designers whose goods are being offered.
So basically they buy from "reputable" wholesalers or distributors. It may very well be that some of their stuff is authentic, but the gamble is up to you. Personally I would never purchase from a site that isn't an authorized dealer of the brands that it sells (you can determine this by visiting each brand's respective website).
^^ Good advice. I wouldn't take any chances either. It's not worth the pain if you receive a bunch of fake stuff or get scammed. Is saving a few dollars really that much more important than peace of mind?