You already solved aggregation once. Asking every merchant to solve agent commerce individually would undo it.
A marketplace already normalises catalog, payment and disputes across a fragmented base. What it cannot yet do is let an outside agent transact against that base under authority the merchant can verify.
Connecting once makes every opted-in merchant reachable without asking one of them to build anything.
Four thousand merchants on your platform.
An agent cannot buy from one of them.
You connect once and the whole base becomes reachable. Platform-wide defaults set the floor: which categories are exposed, the commission model, the fulfilment standard, the returns window.
Inside the platform's floor, every merchant keeps its own ceiling. Opt in or stay out, set its own prices, refuse a category, close for a holiday. The merchant builds nothing.
An agent arrives with a mandate and sees only what the platform exposed and the merchant permitted.
The hard part of aggregated agent commerce is not connectivity, it is whose rule wins. Set a platform default and see which merchants inherit it and which have overridden.
A merchant can always be stricter than your default. It can never be looser than the buyer's mandate.
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A merchant deactivates a listing four seconds after an agent received an offer for it. Marketplaces live with this; agents cannot. Offer expiry and a pre-commit callback fail the transaction cleanly instead.
The merchant never built anything. Its rules still decided what the agent was allowed to do.