I’ve heard of that idea but frankly I’d be frightened by how many people would be parted from their votes by manipulative tactics or people finding ways to buy such votes (even if explicitly disallowed, they’d find an indirect way). That second point in particular would be a big concern because the people who have little else to sell but their own vote would be the ones most likely to sell it and organizations buying such votes would likely be those with a vested interest in keeping the poor, poor which would now be even easier.