Simply have a lsit of who voted up this article, and add a mechanism for changing your vote. It's easy to upvote something that's got a good title, then realise it's crap and not be able to retract your vote.
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!).
In other words, if you vote up a story and then change your vote a day later, the vote has already contributed to putting the article on the front page on that first day, and so the "damage" has already be done.