Yeah, mailing in your absentee ballots together could work - I assume that the people on apps aren't doing that because it would be difficult for most people to travel to another state.
That said, anything that allows you to prove who you voted for is a flaw in the design of the voting system anyway, so to the extent that there are loopholes that allow this, one would hope that they will be fixed.
As for reputation - reputation systems rely on the ability to verify that something has already been completed. If you order something online and never receive it, you are able to complain and cause reputational damage to the seller. In the case of vote-swapping, you shouldn't ever be able to know whether or not the other person voted the way they said they did, so you never actually know that the other person is untrustworthy.