We are just about to launch an online marketplace with a focus on waterfront lifestyle living.
I will stay away from Google rankings aspect. For that, check out the following link: "Busting the Top 15 SEO Myths of 2014" http://www.cio.com/article/2454224/seo-sem/busting-the-top-15-seo-myths-of-2014.html. Good quote from that article though "SEO is a driver of web traffic, not a driver of business"
As one SEO expert told me, search engine optimization is on-going exercise, not a one time event. I would separate efforts between new and returning visitors. Sites in general that get a high number of visitors on a monthly basis usually have a core group of returning visitors that leverage it for content sharing or expertise on a regular basis.
In the case of returning visitors, I suggest having forum/testimonials for people have who have leveraged the website's insights or are subject matter experts and discuss the benefits, challenges, or further application of the information or area. Blogging, linking to articles relevant to the user base, also curating news from social media feeds through sites like Paper.li or Rebelmouse provide an easy content option without much effort.
For new visitors, media exposure and references from top opinion leaders in the space can help. I would look to other portals in the same industry or that attract the same age group being targeting and request the site owners to write a piece, send out a communique and/or reference your service to the targeted audience. Getting local media in your areas to report on your service can also provide exposure. I would focus on finding the free avenues first and when you find the highest referring sites, focus any ad dollars there to get the best bang for the buck.
With any new on-line venture it is critical to build the user base before charging people for the service. The site has to be the go to place for this type of information and discussion to show the foundation value and then adding services that provide additional custom value add. Simple example is Wattpad with over 30M users sharing free stories, in over 50 languages, from any phone, tablet, or computer just recently raised $46M in venture financing