Backstory: Small businesses may not be in a position to hire an internal marketing team or even afford an outsourced one. However, given a known customer lifetime value, a company would be willing to buy a customer for a certain amount of money.
The Idea: A PPC limit buying platform.
Deep Dive: You would build a platform for business owners to set a acquisition cost and details about their company. Marketers would be able to search for companies and ad buy prices. Marketers would be able to apply for the job and once accepted, they would run ad campaigns and they would profit anything over the agreed price.
Validation: This is similar to a lot of marketing consulting deals already done privately, this is putting out a platform for marketers to compete and find new work.
Monetization: You could charge the business once a deal is brokered based on the monthly ad spend. This is a bit tricky since the spend is no flowing through this platform but the idea is to scale the fees based on the size of the deal.