It doesn't seem to be violating anyone's rights, and the Constitution of a free society would prohibit taxes and all other government's infringement of rights, so one could assume that a person buying votes in a free society just want a powerful job.
The question pertains to a free society. If this means a proper society of individual rights and limited government, where government has no powers beyond the three proper functions described by Ayn Rand, I do not see how it would even be possible to "buy votes." The vote buyer would have no special benefits to confer on the payers, nothing to offer in return for their votes, nor any special powers to gain by winning an election. Voting would be purely for the purpose of deciding on the personnel to run the limited government, not on the permitted uses of the government's strictly delimited powers.
The question is actually very ambiguous about what "vote buying" refers to. A clear definition would be in order, which ordinarily would be the questioner's responsibility to provide. But I can give it a try, too.
I tried to look up "vote buying" in Wikipedia, but the closest I found was in the topic of "Electoral Fraud," here. This article contains a brief section on "Vote Buying," which mentions the following situations:
The most famous episodes of vote buying came in 18th century England, when two or more rich aristocrats spent whatever money it took to win. [...]
The first case seems akin to a candidate spending as much as he likes on advertising and campaign publicity to "get the message across" to the voters. The second case -- giving payments directly to voters -- seems more like outright bribery. (The Wikipedia article even mentions one way that a person could provide proof of how he voted, in order to receive his payment.)
I don't think either situation would be likely to become a significant problem in a free society, where there is so little to be gained from it. Objectivist writings generally have classified laws against the first type of "vote buying" (including government-imposed campaign spending limits) as infringements of freedom of speech, and Objectivism opposes such laws.
But I wonder about the second case. Objectivism surely would not condone bribery (except in a foreign country where bribery of government officials may be a generally accepted norm). In a free country, simply publicizing that a candidate tried to bribe voters directly would damage his reputation beyond repair, and again offer little financial benefit to the perpetrator even if he were not publicly exposed for doing it. The cost would far exceed the payback. The moral climate of the country would deal with the issue adequately, without the need for criminal penalties. Such laws themselves would also be fraught with opportunities for non-objectivity, prior restraint, and abuse of government power.
So again, I see the question primarily as an instance of erroneously projecting into to a free society issues and relationships endemic to less free or non-free societies.
Also, mixing the two cases together, as in the Wikipedia article, has the effect of obliterating the issue of freedom of speech (which includes the right to pay for the means to exercise it), by treating two significantly different issues as being of the same ignoble calibre. Objectivism calls this technique "definition by non-essentials" and "package-dealing."
I answered this question in a recent episode of my Rationally Selfish Webcast. An audio recording of my response is available as a podcast here: NoodleCast #79: Live Rationally Selfish Webcast. The discussion of this question runs from 52:00 to 59:40.
My Answer, In Brief: To buy votes in an election would not only be wildly expensive but also likely ensure defeat. It's not an activity that could or should be banned. For my full answer, listen to the podcast!
To catch all the Rationally Selfish Podcasts, subscribe to the podcast feeds in iTunes in enhanced M4A format (RSS) or standard MP3 format (RSS). Or better yet, join Greg Perkins and me for the live Rationally Selfish Webcast on Sundays at 8 am PT / 9 am MT / 10 am CT / 11 am ET.