Hello!
Hope you have a nice day.
I will appreciate if you tell me what industry your marketplace relates to, it will help to give more specific advice.
In general, here are some tips for growing the audience of your marketplace.

Storytelling
Storytelling - this is what will build a strong community around your marketplace and saturate it with the ideas.

You can encourage your users to write blog posts about their experience renting a car and reward those with the best stories.

Partner with other businesses
A partnership is a great way to create a word of mouth for your product and promote it to a wider audience.

Scale the functionality
First of all, listen to your customers. Gather their feedbacks and, when growing your product, primarily add the features that your users request.

Organize events

Industry-related events are a great way to create a word of mouth for your business and unite people around the same idea.