So Tom Perkins—the infamous "progressive Kristallnacht" guy—thinks rich people should get extra votes. This is, I think, mostly an example of how rich people can be dumb and we shouldn't listen to them too much.
But since this is Slate, I do always think it's worth at least considering the idea that rich people should be allowed to buy extra votes if citizens want to sell them. Superficially, the reason we don't let people sell votes is that we don't think rich people should wield disproportionate political influence. Except obviously rich people do wield disproportionate political influence and the prevailing jurisprudence in the United States is that they have a constitutional right to their disproportionate influence. It's just that right now we force rich people who want to wield disproportionate influence to sort of launder their money through a complicated array of political consultants, fundraising professionals, and media firms.
If you cut out the middle man and just let the Koch brothers offer to buy votes on an open market then at least some money would flow to poor people. Right now we know that legislators completely ignore the views of low-income people when deciding how to vote on issues and mostly just do what rich people want. Letting people buy and sell votes would thus redistribute some income without necessarily changing policy dynamics very much.