Glenn- If I have seen further, it is by standing on the shoulders of giants.
Seeking out a channel partner that already maintains relationships with brands/agencies and can position your inventory and offering alongside the rest of their products is a proven model for success- assuming you add value up and down the chain.
If you are able to prove added value for both the brand and the channel partner then you are in business. You may want to reach out to adtech vendors that aggregate and provide brands/agencies access to a range of supply sources (like MediaMath). Happy to make an intro if that would be helpful - message me.
Hey Glenn - glad you're seeking more advice, and hope you get some traction soon. As we've discussed, and to echo Dmitry and Mark below, a person with a good rolodex and solid network clients fitting your target demo is advisable. They can get you in the door, you then have to sell it in.