WHY IT STAYS BETWEEN YOU
A purchase history is a picture of a life.
The privacy question matters more here than it looks, because the alternative solutions all involve publishing something in exchange for protection.
What a buyer buys says a great deal
A record of everything somebody purchased secondhand describes their income, their hobbies, and their circumstances in detail.
What a seller sells says more
People sell things when they need money. A visible history of quiet liquidation is not something anyone would accept in exchange for a safer transaction.
A public chain publishes both permanently
Every settlement becomes a durable record attached to an identifiable party. That is a very high price for solving a coordination problem.
The settlement reaches two people
The buyer and the seller. Not other users, not the wider network, and not us in any readable form.