Job board industry veteran here. You are right it's hard to scale up a job board because of the two sided market. Once you get to a certain size, you are sudden

Job board industry veteran here. You are right it's hard to scale up a job board because of the two sided market. Once you get to a certain size, you are suddenly competing with Indeed, Facebook, Google and LinkedIn (and all the other job boards). Getting started is pretty easy right now because the employer side is very much into pay-per-click or pay-per-apply syndication and there is high demand for jobs on the job seeker side of the market. The key is to have a source of traffic that you can convert into click and applications. You'll see all kinds of niche job boards spring up right now fueled by traffic from communities like this, product hunt and even appropriated email lists that people take when they are laid off (yes, that's probably unethical, but lots of job boards get their start when a developer or recruiter starts one up in anger).