If a person is willing to sell a vote for cheap, then they are better off curating because they would make more money.
They wont be selling it cheap. They would be selling it for a lower amount of liquid funds but for more SP.
Whatever they would get from vote selling would offset the loss of being ineffective curators from selling votes.
They still get curation plus whatever they get from folks paying for votes meaning that their behavior would not change.
On the other hand, due to vote buyers having to pay less for the same $ amount on their post the demand might actually increase.