How would you prove that you voted how you said you did?
If you took a picture of your ballot, or even if you filmed yourself putting it in the envelope and putting it in the mailbox, there's nothing stopping you from taking it out later, tearing it up, and going to vote differently in person.
Just do it in person. The voter fills out the ballot in front of the buyer, seals and signs the envelope, and hands it to the buyer in exchange for cash. The buyer then puts it in the mail on the voter’s behalf.
The voter could go to a polling place afterwards and attempt to cast a provisional ballot but my understanding is that this is difficult, varies significantly state to state, and in many cases is not possible given that mail in ballots are detached from the voter identity ahead of Election Day in many states.
First, this is too much trouble and many won't do this, second, you can lie to people that you have the means to verify their vote, third, you might require a person to write a code word on the ballot so that you can verify that they actually casted that ballot.
If you took a picture of your ballot, or even if you filmed yourself putting it in the envelope and putting it in the mailbox, there's nothing stopping you from taking it out later, tearing it up, and going to vote differently in person.