I'm looking at the problem through the lens of "why does a voter want to see their ballot". The answer which prevents the issue of vote buying is "to audit the validity of the vote", which then is ensured through putting some stop-gaps in front of viewing the vote in the form of requiring intervention from the entities organizing the ballot.

Ie, if a malicious entity wants to make sure that the votes they have bought are corresponding with what they asked, they need to go through a more difficult process than just asking the people they bought from to reveal their vote.