https://techcrunch.com/2017/07/14/adquick-seed-funding/
We still have a long ways to go, but I think there's a good chance we'll prove you wrong :)
We work with sales people daily and getting their inventory to our customers. They love that we are bringing them new business. As for brand guidelines, we're in constant communication so this isn't much of an issue.
If that's true that's great but it's the opposite experience of what I've experienced with large media companies. They pretty jealously guard their inventory and hate the idea of transparent pricing. That's great if you haven't experienced it yet, but I suspect you will at some point. Best of luck to you regardless. I'm rooting for you.
As for AdQuick (and Fliphound for that matter), it seems like you're just another middleman. There is no bidding. Am I missing something?
Now that ad prices have more sanity and relative value across mediums, now may be the time to do this.
I see this as an idea that will happen, it is just a matter of when.
I'd start buy building a big data model to value billboard space. Estimate the eyeballs using traffic data, size, visual competition, etc. Sell the analysis, broker space, and then in 5-10 years you could build the Adwords model and eventually own the whole damn industry. You could also sell the company to an Ad Agency at any point.
It has legs, you just need serious commitment from an owner of inventory (physical boards), which are controlled by 3 or 4 organisations.