This site made the news a while back, an underground marketplace hidden more or less in plain sight. I happened to think of it again when a friend of mine who lives a couple of thousand miles away was trying to source a few difficult to find items. The link I have is:
Bitcoins, a digital currency which I presently know next to nothing about, except that some of the prices seem a bit high. As of today, a Bitcoin trades for $2.77 USD.
Sellers receive ratings from buyers, which if accurate would help to prevent rip-offs. Without ratings I'm not sure such a site would be practical. Given reasonable ratings, most of the risk involved would be in receiving the package itself.
I have not used the site myself, but I find it very interesting.
I have a certain amount of libertarian sympathies in me, and I would tend to agree with one highly-ranked seller's viewpoint on this kind of thing:
About me: Cautious, normal, everyday person who ships high quality products discreetly. I believe that the use of drugs is a personal choice. I also believe that those who are able to navigate the complexities involved with acquiring Bitcoin and configuring Tor are intellectually fit enough to research their drugs and take appropriate doses relative to their experience and comfort level. Try visiting erowid.org as a nice informative starting point if you're unsure.