We are looking to build an integration for merchants who already use PayPal to process transactions. This integration needs to get information on those merchants' transactions via some mechanism, likely either the Transaction Search API or via webhook. How can this be achieved?

After reading through the overview, the docs seem written from the perspective of pulling data from one's own PayPal account via the API. How can I pull data on behalf of many different merchants as a 3rd party? Specifically, how would the OAuth work?