Yes, creating a mockup or prototype of your marketplace idea before building it would be very beneficial for several reasons:

Validate the concept: A mockup allows you to simulate how the actual marketplace would work and look like. You can test it with potential customers to validate if your idea resonates and solves a real need. This is much faster and cheaper than building out the full product first.

Work out usability issues: Putting a mockup in front of users, even if limited in functionality, can uncover major usability issues early on - things like complicated flows, confusing navigation, etc. These are much easier to fix in a mockup stage.

Clarify requirements: Interacting with a mockup, even if it's static screens, forces clearer thinking about detailed functional requirements. Things like exactly what data and features are needed on each screen becomes more obvious.

Estimate effort better: A mockup gives developers and engineers a much clearer target to estimate effort against, rather than vague specifications. Things like number of screens and integrations required becomes tangible.

Improve internal alignment: Walking internal stakeholders through a mockup aligns them to what is being built much better and gets their feedback incorporated early.