My understanding is that larger companies with bigger ad spend don't have time to hand-pick each app or web page. (Using human labor to pick apps and pages can significantly raise the cost I presume.) Perhaps there is a marketplace for smaller advertisers here.
I was thinking in terms of a more human approach where advertisers are actually looking to connect with their potential users.
App publishers don't sell impressions - they sell influence by allowing the advertiser to get a spot from their app real-estate.
It's certainly possible, but not scalable. If your goal is to support your own work and you have the hustle and editorial eye, you can make money.
Keep in mind that the big players can wipe you out in a second. Overcast exists because Apple's benign neglect of podcasts. They can wake up at anytime and wipe the app and ad market out.
I'm not sure I've understood - what's not scalable? Selling ads by yourself or using a network that helps you sell ads?