I think there's a fair chance that Assassination Markets are a libertarian construction that underestimates humanity's inherent valuing of life.

As a thought experiment it is interesting, and I'd encourage anyone to read the actual essay (http://jya.com/ap.htm) but, and feel free to call me optimistic, pessimistic, closed minded, soft, or whatever your world view leads you to think of my opinion, the idea reeks of an academic exercise that would just not be accepted universally enough to work.