I think the "easy" way to shove yourself into the market would be to tackle a niche, so you can specifically target both the advertisers and the publishers that

I think the "easy" way to shove yourself into the market would be to tackle a niche, so you can specifically target both the advertisers and the publishers that have an audience that would engage with those advertisers. So if you decide to choose, say, online tackle shops on the advertiser side, you'd target fishing blogs on the publisher side. That way you're pretty focused on a niche and don't have to solve difficult ad matchup/inventory problems early on. The bonus is that if you figure out how to make publishers and advertisers money, you're going to make a lot as well :)