Transactions have always taken place。 For hundreds of years that 'place' was a market or， more recently， a shopping mall。 But in the past two decades these physical locations have increasingly been replaced by their virtual counterparts - online platforms。 Here， author Michael C。 Munger demonstrates how these platforms act as matchmakers or middlemen， a role traders have adopted since the very first exchanges thousands of years ago。 The difference today is that the matchmakers often play no direct part in buying or selling anything - they just help buyers and sellers find each other。 Their major contribution has been to reduce the costs of organising and completing purchases， rentals or exchanges。 The Sharing Economy: Its Pitfalls and Promises contends that the key role of online platforms is to create reductions in transaction costs and it highlights the importance of three 'Ts' - triangulation， transfer and trust - in bringing down those costs。