As the others said, it can vary but I have found that one of the fastest ways to grow an audience is to give away some related knowledge. For me, this was creating online courses to provide for free in the areas of photography and online marketing. It does not take a lot of time to do a screencast or a couple of videos teaching something and there are plenty of marketplaces for online learning that have students wanting your content. From two courses I have just shy of 30,000 students and it's still growing. Once you have a decent audience who you trusts you because of what you have taught them, you can ask them to check out your offerings at other marketplaces.
It depends on your niche, industry, what your goals are, who your market is, and so on. There is no "one size fits all" answer. Feel free to set up a call to discuss your needs and get "real" answers instead of dangerously ineffective cookie cutter answers.
The beauty of a "two-sided" marketplace. I can actually tell you how we started ours. I actually wrote a post, but in essence we "seeded" one side of the marketplace with our own video courses. This built confidence "enough" to the point other authors were "enticed" to upload content to our site. Today we have more 500,000 registered students and receive about 1M uniques per month.
Glad to hop on a call and dig deeper or just read my post on how we started: http://blog.tareasplus.com/startup-leveraged-youtube-create-marketplace-500000-subscribers/
Focus on one niche and find out what your audience values the most. Then, build content around using inbound marketing. Remember, SEO rules have changed. So you can't just take content from another site and republish on yours. You'll actually lose points doing this and your ratings will suffer.
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 - 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.
Industry-related events are a great way to create a word of mouth for your business and unite people around the same idea.