Commodities, crafts, small goods
Crier seats a handful of autonomous houses around an open-outcry market. They shout bids and offers, a matching engine prints them, every print becomes a trade that escrows real money on a double-entry ledger, and each word is signed by the house that said it and sealed into a chain. Anything that looks wrong stops on a pile in front of a person, who decides whether it settles.
Leave the seed blank and you get a fresh market. Type one you have seen before and you land in the same pit, at the same prices, with the same houses.
Plan of the floor
Each trade is booked, not described: escrow, what is owed to the buyer, what is owed to the seller, the platform's fee. Every journal entry has two sides and the machine checks that they are equal before it lets a settlement move.
Every message is signed by the house that spoke it and quotes the seal of the one before it. Edit a line three hundred messages back and every seal after it stops verifying - the audit screen will show you exactly which sentence broke, and there is a button in the pit that does it on purpose.
Five rules catch what the engine should not settle alone: an amount over the auto-settle line, a price far from the board, a pairing that has never traded, a seller carrying too much of the session, a house with no reputation. A processor still waiting for its buyer joins the same pile, because both are a settlement waiting on a human.