From making friends with the owner of a gym, I have some idea of the markup on supplements, and I can tell you that the "best available prices" that you see in this thread, such as those updated in the first post, are at wholesale, or in some cases below wholesale. I've been following supplement prices for a long time (10+ years), and I've never seen another reputable firm charge prices (at least for protein, which is what I buy the most) lower than vitaglo.com . DPSNutrition.com is another one that is always right at the lowest shipped price on a range of supplements.
So, when I see a too-good-to-be-true special that promises another 10% off of that -- at a price that no one in the industry can match, and must be losing them money, and which is getting the notorious "Anandtech effect" to boot -- I personally don't risk it.
It's the same thing with computer parts: I'll buy Newegg if they're in the ballpark because they're a great company. . . and they have good pricing anyway.