In most of the secular West, buying votes violates statute legislation. Quite apart from being an unenforceable contract, it is a baldly unethical agreement. It's also one of those rules that rubs the natural law the wrong way. Folks have strong moral intuitions that buying votes is just wrong. Voting represents pure civic-minded ideals, uncorrupted by the vices of the market.
Now, you may or may not be sympathetic to the suggestion that it is the overlap between governmental authority and the functions of trade that seem to generate the most corruption, but I think you'd have to admit that the fairy tale of democratic participation is a cornerstone of Western Civilization.
So why the moral outrage? What's so all-fired corrupt about vote-buying? Suppose you've got a fair chunk of the population that's basically indifferent between Candidate A and Candidate B. Why not just pay them directly to basically flip a coin instead of usurping their scarce attention with political advertising? Yes, all else equal, the wealthier candidate wins, but how is that all that different than what we've got now? Politicians all but bribe the public with promises over the disposition of their own money anyway, so how is this any different? Under what precepts are campaign promises more euvoluntary than vote-buying?