The machine gives me a paper record of my vote to keep. Meanwhile, it appends my vote to the blockchain. I go home and use information from my paper printout to verify my vote is indeed counted and on the blockchain. If my vote isn't on the blockchain, I have a paper record of it that can be used to append it, should the machine "fail."
A) Person stands outside of polling station in swing state/district, offers $5 for a verified vote. Walk in, get your receipt/blockchain entry/doodad, show it to the vote buyer outside, get $5. Not at all a good situation, because it enables votes to be bought.
B) Person stands outside of polling station in swing state/district, offers $5 for a vote. Walk in, vote for whoever you want, then walk out and pocket the $5. Because the vote cannot be verified, they have no proof one way or the other.
Better to have an adversarial system, where any votes coming out of the ballot box are observed by representatives of all candidates. Each candidate has an incentive to ensure that the others do not tamper with the vote.
Scenario A could still happen, but seeing as it's explicitly illegal, the likelihood of it causing widespread voter fraud is unlikely.