Which is trivially easy with the availability of camera-phones (just include your ID in the shot). I know it's not foolproof as you could request a new ballot,

Which is trivially easy with the availability of camera-phones (just include your ID in the shot). I know it's not foolproof as you could request a new ballot, but I'm guessing those buying votes aren't the smartest folk. Plus you'd be crazy to not accept the money upfront, as there's no way they're actually going to pay out after the fact and it's not like you can take someone to court for not upholding their end of an illegal bargain.