It's not so much selling votes, as to discourage voter intimidation: The husband forces his wife, the boss forces his subordinates, the local mafia forces their victims to vote a particular way.
I don't know the rules in the USA, but in the UK it is generally forbidden to stream yourself voting or take a photo in the polling station. Maintaining the secrecy of ballots is high priority.
Someone might still privately take that picture of their ballot paper, after all there's a private voting booth; the officials wouldn't know. You're allowed to say you made a mistake and ask for a replacement ballot paper, so you could show your boss the version they want to see, and then vote differently.