It would, of course, be completely inappropriate for me to write that if you're interested in illegal prescription drugs, you should go talk to Stefan Savage of UCSD. But it's more appropriate for me to give a shout-out to him for his recent appearance on Planet Money's podcast, in Episode 430: Black Market Pharmacies and the Spam Empire Behind Them, which I listened to on my way into work this morning. It's well worth listening to. The podcast's take summarized (which matches my experience with other non-drug online advertising based businesses, and I think I'm summarizing Stefan correctly) -- it's actually a pretty boring business. Online pharmacies are just trying to sell something, get their cut, and spam mail is their effective way of advertising. (Since selling prescription drugs without a prescription in the US is illegal, they can't exactly advertise on TV.) Stefan mentions in the podcast that in cases he's examined, they don't appear to be trying to send you placebos or bad product; they're just making their margin, like other retail businesses.
Maybe Stefan will see this and offer some pointers to research in the comments. In any case, I enjoyed listening to his insights and smooth-sounding voice on the drive.