Individuals can see their own vote, and who they voted for, right?

If that's the case, it would be evidence that an individual voted for a specific candidate. This is, essentially, proof for anyone who'd want to purchase votes.

If they can't see who they voted for, they'd need some alternative way to verify that their vote counted for the candidate they wanted to vote for, without actually disclosing who that candidate is. Which would add some complexity to the solution.