Actually, when talking about possibilities for election fraud in contested places, threats seem much more common and plausible than simple votes-for-cash trade. It's more widespread in places with less democratic traditions, but USA history has also a fair share of examples.

The most common current method worldwide for "vote buying" seems to be done by large regional employers organizing voting as such for their employees (time schedules, transportation, lunch/party after voting), sometimes that can be very effective as strategic voting (i.e., bussing large numbers of voters to a different district). If the votes would be actually verifiable, then even without any illegal violence it'd simply mean near-permanent unemployment in that town for "wrong-party" people.

And that's about ordinary elections. For a different example, the recent referendum for Crimea joining Russia, according to their official results, had ~32000 people voting against. I'm not informed about what's happening in Crimea now, but I'm very sure that if I was one of them, I wouldn't want that vote to be verifiable - the hypothetical risks far outweigh any possible benefits.

Even "ordinary" elections have a similar risk to Crimea in some parts of the world - if I lived in the Basque Country in Spain, and voted for the (mainstream, right wing) Partido Popular I wouldn't necessarily want that to be published.