I bootstrapped a marketplace to 6M+ users. It’s startups on hard mode.
We focused on sellers first but the hard part is no matter what side you choose, you have to artificially inflate the other side.
That costs money, time, and a sick amount of faith it will all work out.
When starting a marketplace, focus on getting more of whichever side is rarest, which is almost always buyers rather than sellers. (If it's sellers, you've discovered a gold mine.)