It's easy to configure the voting booth to provide privacy for the voting itself, while making it risky to photograph the vote. You don't know the vote was actually delivered, ex. he might have gotten a fresh voting paper or punched/stamped excessive options to annul the vote. There is also an electronic evidence trail of the fraud - any caught vote seller will immediately cooperate with the authorities and rat on the ringmasters.
Real world security works on "just hard enough not to be worthwhile", not absolute guarantees.