You can't use IAP to pay the other person for services rendered outside of the app. You can use Apple Pay.
But some of what you are selling is enabled through the app - like the search for a reviewer or selecting the reviewer. Maybe even some of the services are using app functionality. If you wish to sell that then you divide the sale into two components - one is IAP and the other is Apple Pay. So a user might upgrade their app so they can see all reviewers' qualifications or a reviewer might upgrade so they can advertise their qualifications or a user might.... All that would be sold through IAP since it is part of the app code. Once the reviewer and user are connected, the reviewer would charge and get paid through non-IAP systems.