Usually sites like this and Bluefly.com sell their items cheaper than retail because they get their items directly from the factory, and are able to sell them at a small discounted price.
Here's a quote from their website's Term and Conditions
Guarantees and product price indication
PRET-A-BEAUTE.COM sells authentic products.
PRET-A-BEAUTE.COM offers for sale luxury and top-quality products. These products are purchased by PRET-A-BEAUTE.COM directly from fashion houses, manufacturers and dealers, carefully selected for their stringent quality control.
PRET-A-BEAUTE.COM does not sell second-hand products, flawed products or products of lower quality.
It is illegal to claim something is "authentic," when its not true.
And some sites get away with this because they state one thing, but in their Terms and Conditions they state another.