* 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.
This was win-win for all. I made tons of sales from there.
So my suggestion is to work not on website that is good for ads, but on website that is good to deliver information and answers on questions. Ads and referral sales will come after that.