You have a controlled environment for the workers, who do their work on locked down software and hardware. Basically, employees don't get to bring any personal possessions into the room where they do the work, the software through which they do the work does not allow them to copy or export any data, the OS that hosts the software doesn't allow them to do anything but run the software.

Well, I guess I was assuming they were tackling that problem or had some way of addressing it. That assumption was based on trust, evidently misplaced.

It does seem like a hard problem to truly solve. The difficulty of doing the crime is so low compared to the difficulty of excluding anyone who might feel slightly inclined to commit it that it seems inevitable that removing humans is the only viable solution at scale going forward.

What "illicit market" could possibly be interested in a pile of random receipts?

Receipts would have any value if all of them are from the same person and can be used to reconstruct that person's activity. And even then it'd be pretty damn tricky to sell that "file" as it still has no exploitable value.

You figure out a way to remove the name / identifying info and just have someone classify the transaction.