We actually just released a “proposal” feature that helps you sell directly to an advertiser at a lower commission rate: https://buysellads.com/sell/proposals
The thinking behind the 15% is:
– 3.5% or so for payment processing
– the rest for us handling all support, billing issues, and providing the advertiser with a place to manage the ads on their own, see stats, etc.
Would love to hear any thoughts you have if/when you check this out.