[PeterisP]

Voter coercion and retaliation for voting 'the wrong way' is probably even more important than vote buying/selling.

A key feature of a secret ballot is that it must remove the ability for anyone to verify how you voted even with your cooperation (no matter if willing, coerced or bought) - you must have plausible deniability i.e. any reasonable "demonstration" to others how you voted must be possible even if you actually voted differently.