Legislation varies across different countries. When you hold other people's money you become a custodian, in the early days of a startup you can do it without a license, but at some point you would probably have to become compliant. That's why for early stage startups I recommend using payment gateway providers like:
- PayPal Commerce Platform for platforms and marketplaces: https://www.paypal.com/en/business/platforms-and-marketplaces
- Stripe for marketplaces: https://stripe.com/en-es/use-cases/marketplaces
- MANGOPAY (only in Europe): https://www.mangopay.com/pricing/

All these providers count with licenses and compliance teams to ease you the task of having to deal with it on your own.

I'll be happy to share with you some examples of what I've done with my startups and with the companies I consult.

Hi! Agree 100% with the rest of the answers in this thread. If you need some help to start with, I recommend you to have a look at the methodology Lean Marketplace, check it out for free here: https://theleanmarketplace.com/

Good luck!

Hi! I am a marketplace entrepreneur myself (Sharing Academy) and have helped dozens of sharing economy entrepreneurs as well (Lendi, Truke, Mentor Alliances...). What worked best for me was convincing the users to become team members.

Our best programming tutor in Sharing Academy became CTO, his best student in the platform became his intern. Another student became head of our customer care department, and so on...

Build a great product, engage wit your users and make them become part of your movement.

Read The Lean Marketplace (theleanmarketplace.com). It is written by my mentor and friend Cristóbal Gracia. It helped me start my own marketplace and I use it's methodology to help entrepreneurs, startups and corporations launch and scale their marketplaces successfully.

There are many ways you can make a low transaction volume marketplace profitable. For instance you can charge not a transaction fee of the low volume transaction, but an exclusive membership fee to join the community, or a lead generation fee, or partner with a strategic partner that sells desired products for your users, etc.

Happy to give you more examples if still interested.

I suggest you pick a freelancer's niche that has difficulties gathering together (either online or offline). Then you create a place where they all can talk and engage with each other to share best practices. Finally you offer them the opportunity to make business in that community.

Let me know if you have further questions.