Click through one whole deal the way the planned app will work: start a deal, name an arbiter by their address, invite the other side, pay into escrow, deliver, then release the money or open a dispute, and withdraw. The arbiter hears about the deal only if there is a dispute.
Prototype. Only the screens are real. Nobody signs in, nothing is sent anywhere and no money moves: the people, the payment and the passing days are simulated in your browser. Reloading the page starts over.