Allow vendors to create listings and clients to create requests for projects based hourly rates, along with the ability to invoice / bill multiple times per project based on a certain number of hours worked.
For example, a client hires a vendor at $50 a hour for a project. The vendor works 10 hours in one week and submits a bill for $500. Project stays open the next week, and vendor can submit another bill at the end of that week, and so on. At any point, either party can decide the ongoing engagement is over.