I don't think vote for sale (however promises during campaign are equivalent to buying people votes) is a big concern in the current system, but rather threat against people . That's why it is anonymous, so people can chose whomever they want without being threaten.
But again, as I said in a previous reply, the election is just part of a decision making, a way so people can express their concerns over the leader who will govern them, or some aspect of the laws to which they will abide. In my opinion this is more sharing the responsibility (and all which goes with that) on who gets elected and the way he will govern than anything else.
I personally don't think that negative voting would change anything except the math and the strategy to win an election.