There are not really elections to buy.

China has elections. The idea that multiple parties is what democracy is is a myth. In fact usually its the opposite. Most neoliberal democracies are essentially one party states with the illusion of ideological disagreement. The United States is probably the worst in this way. They two parties in America differ on a few wedge issues the capitalist class will allow them to differ on (ones that matter a lot to the people they effect to the point that a lot of people get very invested, understandably so), but unite on foreign policy and most aspects of capitalist hegemony. Because thats the thing, in America (and the rest of the liberal democracies) the capitalist class are the ones in control and they use money to buy election (once again, America is probably the worst in this sense, but its not THAT Much better in the rest of the first world). In China the rich might have money, but they can't buy the politicians. They have no influence on policy.

And I say all that as someone who essentially agrees with you that its bad that China has billionaires.