* It's time consuming for the seller and buyer.
* You need to give the buyer reliable statistics. Page-Views, Click-Trough rate, demographics and what not, before and after you sell the ad spot.
* You need to define the price, which is not that simple.
* You need to provide the technical means to manage and display the ads.
* You need to prevent abuse.
* You need a contract for each buyer.
* The buyer needs to trust you.
* You need to trust the buyer.
I think that ship has long sailed. It would be tremendously cool, if there was some kind of self-hostable ad software, which takes care of the technical details and would act as middle man, connecting buyers and sellers. Some kind of open marketplace based on open software. But I doubt, this will ever happen. Would be a cool thing to disrupt the current industry though.