if you're looking for bargains, bluefly has lower prices although the selection isn't always great. Once in a while there are some great finds though. I've also noticed a disturbing trend in that bluefly is starting to add more and more mainstream crap (like diesel, etc).
Interesting you say that about Bluefly. I noticed with their newly redesigned website they are trying to become a bit more upmarket in their offerings (more Helmut Lang, Costume National, etc.) They do have their share of Diesel, Levis, etc. Plus you can always find a 15% off code for Bluefly, but Yoox features a much better selection. Yoox can be trusted as I have had friends rave about them and order from them without incident. Also check out Smartbargains.com and Overstock.com They feature some Lang pieces (not the greatest) Movado watches, etc. Good stuff to pick up and resell if you are into that. Shipping is dirt cheap as well.