… is from page 44 of Nathan Oman’s 2016 book, The Dignity of Commerce (footnotes deleted):
Exchange has two key features. First, it is unanimous. Each party to a market exchange has at least the nominal power to veto the transaction. In contrast, political systems allow action in the face of dissent. In a political debate my goal might be to persuade, but politics provides a mechanism to triumph in the face of my interlocutor’s opposition. Democratic outcomes are always premised on the absence of unanimity, although differing decision procedures require greater of lesser levels of consensus. The wishes of at least some members of the polity can be ignored. Not so with exchange. Without the cooperation of my partner, no exchange is possible.
Second, exchange requires that each party pursue the other party’s interests to achieve his or her own interests.