I would not start a remote job board today. There are just too many competitors in this niche. But you know what might work?
A remote job board aggregator.
Imagine if you could aggregate all the remote job boards into one search engine? I think that could be a nice lifestyle business by providing aggregation as a value add to seekers. You could scrape the boards for free to get started and eventually offer remote job boards the chance to be featured themselves.
You could sell advertising to seekers (like resume services) as well once you start building up an email list and eventually add things like Adsense to the site itself.
Curating jobs like this helps job seekers save time so that is what you’d be bringing to the market with a site like this. As far as I know I have never seen a niche aggregator like this but it certainly is worth exploring.
