Hi
This is a classic market place SaaS (Software as a Service) and the classic answer for getting both sides is quality content, quality marketing, and word of mouth (which you'll get once you provide quality service).
The key issue, BEFORE you think about how to get them to your platform is to first create a POC (proof of concept) to make sure that people want to use such a marketplace - doing this is fairly easy and will save you hundreds of hours, and tens of thousands of $ building the website.
I've helped numerous startups working on marketplaces and would be happy to advise you on doing the POC (and connecting you to a development company if you want one). I can also help you create the basic landing pages for the POC.
Good luck
I've successfully helped over 350 entrepreneurs, startups and businesses, and I would be happy to help you. After scheduling a call, please send me some background information so that I can prepare in advance - thus giving you maximum value for your money. Take a look at the great reviews I’ve received: https://clarity.fm/assafben-david