For every assassination bounty hosted they should also host a corresponding anti-assassination bounty. The assassin would be paid the net pro-assassination value, that is, the difference between the two bounties, and the bounty hosting site would keep the remainder. For opposing interests of equal magnitude in a bidding war this would be hugely profitable for the bounty hosting site and also result in nobody actually getting assassinated. It would also be more equitable because it represents the opinions of both pro-assassination and anti-assassination sides, not just the pro-assassination side.
Though seriously, the entire subject is revolting. Almost every American, love Obama or hate Obama, love Bush or hate Bush, agrees that they do not want their President to be assassinated. Despite disagreements in American politics, there are essential fundamental core values which unite us all, and that we do not assassinate our leaders is one of them.