A key feature of a secret ballot is that it must remove the ability for anyone to verify how you voted even with your cooperation (no matter if willing, coerced or bought) - you must have plausible deniability i.e. any reasonable "demonstration" to others how you voted must be possible even if you actually voted differently.
And for context on the scale of this, in the 2020 election there were 65,642,049 mail-in-votes cast. And the outcome of the presidential election was decided by 42,921 votes. [1]
This keeps coming up but is not generally true. See my comment from another thread [0]:
I don't know how it's done in the USA, but in Germany voting by post has to be carried out before the day of the election. The actual postal votes are stored and only opened on the day of the election. After somebody send in their postal vote they can go to the public voting office and declare to invalidate their postal vote. The people counting the postal votes will get a list with invalidated votes and remove these envelopes before the votes are opened. The person who invalidated can then either do another postal vote or vote at the ballot box.
So in Germany postal voting is secured against selling votes.
Which is why mail in voting is a bad idea
Just because bad things are happening doesn't mean people need to like more bad things being made easier to achieve