Depends - what are you trying to do?
Mockups can be good for raising an investment (although not easy to do without an MVP and some data). They can also help the developers better understand your needs. But it all depends on the type of platform, who is doing the coding, your budget and numerous other factors. I built a MVP version of a marketplace without mockups because the design layout was fairly easy to understand - so no need. But again - it depends what you're building...
If you have the money, and someone you trust, it can't hurt - but first understand why you need it.

Good luck

I'm a mentor on Startups.com, an international lecturer on entrepreneurship and negotiations, a commercial startup lawyer and a leadership coach. I've successfully helped over 400 entrepreneurs, startups and businesses and I would be happy to help you.

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.