I note that perfectly legal bookmakers in Ireland already accept bets that President Obama will be assassinated while in office. link

It isn't precisely an assassination market, because people other than the assassin can join in the bet, diluting the odds.

But, yes, I take your point. With a single central authority, such bets could be rejected as invalid. With multiple authorities, it would require all of them to reject it. On the other hand, authorities that don't reject such bets as invalid might end up with a poor reputation for reliability.