And the person voting wants to make sure that when they voted for X, that the machine counted that they voted for X.
I think the compromise between these 2 would be that the receipt is very clear who you voted for, but you have to place that receipt into the ballot box in order to cast your vote. This way there is no possible way to prove to your blackmailer that you voted for X. Also, the machine could be able to deal with disabilities (blind? macular degeneration? need lots of time to actually vote?).
I'm sure I'm missing something here. I think that one would want it easy to verify but impossible to "sell" to a 3rd party. If it is encoded into a barcode, or hash string, the average citizen won't be able to verify any of it.
Vote selling/blackmail is possible by taking a picture of your ballot while in the voting booth. One way to reduce that threat is to allow people to create as many ballots as they want while in the booth.
And to restrict people from using their cameras while voting, so there's no way to connect it with the ballot actually deposited. (Yes, they can start recording while in the booth, but what happens before/after the recording period.)