Which type of users is essential? Who should you focus on to acquire first?
For any Marketplace platform to function & grow well both sides of the user base needs to be developed in tandem. Initially depending on the service the Marketplace is trying to provide, it is important to focus on users that will provide the service.
Uber: Drivers, Clarity: Experts, eBay: Sellers, UpCounsel: Lawyers etc. At the same time these service providers need optimum infrastructure, tools etc to showcase their services in order to attract consumers. Once you have a good number of service providers, you should begin promoting & targeting consumers. Without consumers the two-sided marketplace will not function.
Though you can begin focusing on the consumers later, it might make sense to begin targeting a small sample of users so they serve as initial users/beta testers of your two-sided marketplace.
Hope this helps. Happy to connect further over a call to get into specific strategies.