[iambateman]

Buysellads is very close to this, and has been around for a long time. I ran beaconads, their subsidiary, for a short time and the biggest issue is that most ad buys are coming from a handful of massive companies.

Old Navy doesn't want to deal individually with a site that doesn't have 10,000,000 visits/month. So we end up needing an intermediary at the same scale as the big advertisers.

Not saying this can't work at some level, just that there are some big economic forces fighting against scaling a product like this.