Highly recommend you check out OnSched.
Our platform is built specifically for directories, marketplaces and aggregators. Companies who need to build booking that allows vendors to operate independantly with a fully exposed API.
Our "Top-down" architecture gives you the ability to manage an unlimited number of vendor accounts, manage all admin and backend settings through our Business API and build custom booking using our Booking API (or our pre-built embeddable widget).
Completely white-labeled platform and have many use cases of online directories live. I'd love to help!
Check out TechnologyAdvisors.com to see an example of it in action.