I think the concept you're looking for is "voter-verified paper audit trail", which is supposed to take place while in the voting booth. https://en.wikipedia.org/wiki/Voter-verified_paper_audit_tra...

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.)