EBay's marketplace isn't a medium with a 2 in the middle of it. It's a place where people do busines with each other. Not to each other. Nor is it a performance center. Call it a constituency, a community, a web of trust or just a good place to do business.
Traffic jams are sticky too, and good for nothing but billboards.
What you're selling isn't just advertising. You have always valued that trust more highly than anything else. That's because eBay has the soul of a marketplace. Not a medium.