Social Transactions is an application that allows communities of consumers to collaboratively sense the market from mobile devices, enabling more informed financial decisions in a geo-local and timely context. The mobile application not only allows users to perform transactions, but also to inform, share, and purchase in groups at desired times. It could, for example, help people connect opportunistically in a local area to make group purchases, pick up an item for a friend, or perform reverse auctions. Our framework is an Open Transaction Network that enables applications from restaurant menu recommendations to electronics purchases. We tested this with MIT's TechCASH payment system to investigate whether shared social transactions could provide just-in-time influences to change behaviors.