Currently, each person is left to give their own 'meaning' to a vote. I think that this approach would help define that meaning: A 'vote' is public support for the quality of an article, such that you are willing to associate yourself with it.
As an aside, I wonder if each of article submissions, comments, and voting could be thought to respectively show that a person is interesting, intelligent, and has good judgment?
It would be a weird scenario indeed if vote-buyers were only able to pay out for stories that did not get a positive score.
Vote-buyers could still gain some long-term information that would enable payment, such as the reliability of a given paid-voter over time (by checking the stories that didn't make it), but that same information would be available to the news.yc admin(s), who could then just ban the offending accounts (even if they weren't suspected of vote-selling--just for consistently supporting crappy stories!).