There're over 36.000 ad network that uses google's adwords service. But I'm not sure that google accepts this kinda service.
Adwords definitely does not accept my business (I checked!). I think your suggestion of creating a digital agency would be workable if they did; we could even go direct with google if they did. I've come across some big adult website ad networks, but have found them to be surprisingly archaic with payment types and feature sets.
This is one of those "Shut up and take my money!" experiences!
Buy or collect adwords/facebook coupons and advertise customers' websites with your personal account, but I think you need to be adwords certified or at least you need to become a verified advertiser.