Someone can offer me $100 to vote for Candidate A. Because I can’t prove I voted for Candidate A, and they can’t prove I didn’t (unless Candidate A doesn’t get any votes at all), their effort is wasted.
Therefore, it doesn’t matter what someone offers to someone else for their vote, so long as the ballot remains secret.
Yes or no?
Uhh…no, it should not be legal.
Just because there’s no way to prove you did vote for someone doen’t mean you didn’t. The point is that the intent was there for someone to pay you to vote for their candidate, and you accepted to do so, even though in the end you might not have.
If I’m a hitman and a man hired me to kill his wife, if I take her money and agre to do it (even if I never actually do) I’m going to jail if I get caught. Hell, at the very least I’m guilty of fraud. 
Only if you want to have a fair system. At present, voters make their choice in part based on what candidate promises them in terms of benefits. If you allow bribes you take the promise part out and the winner of the election is whoever has the best system for bribing people.
If you can allow bribes, can’t you allow other incentives to sway people’s votes? Like, say, “If you vote for Candidate A, I won’t beat the living shit out of you.” As long as we have a secret ballot, they won’t know if you did or not, so do you feel comfortable taking your chances? We already had this system and decided it was no good.
In one of his books, Tip O’Neill (the late Speaker of the House) wrote about one method used by party machines to ensure that people voted the way they “should”:
As a voter came near the polling place, he was met by one of the party operatives, and given a ballot with the “correct” box already checked. In the polling place he was given another, blank, ballot from the official. He cast the one that the party operative had given him, and gives the blank one to the operative on the way home. This proves that he had cast the pre-marked ballot, and the operative marks the new blank ballot and gives it to the next guy.
Ed
But for this scheme to be effective, you would have to live in a banana republic. I don´t think that any western democracies would allow this. If a significant number of voters is affected, it would be more than obvious, and there are sureley laws that forbid this, and they can be enforced because it´s so blatant.
It was Standard Operating Procedure in Boston in the 50s, according to O’Neill. At that time there wasn’t much federal oversight of municipal elections.
Ed