To serve the ads under the 1st party domain - without ending up in a situation where the entity reporting the number of ad impressions and the entity receiving money based on that number of ad impressions are the same - is often significant effort.
Either someone buys it or they don't - at some price point someone will buy this (Hell I'd personally buy it at some price).
This is what online advertising should be. No impression charging, no targeting, no tracking (and of course no automated fraud prevention)
Today, when tracking and targeting is possible, obviously dumb online ads are hard to sell. Why would I pay $10k to show my shoe banner to everyone if I can pay $0.01 per impression to 20-30 year old shoe geeks only?
If only dumb ads are possible (as in print media) then they will be possible to sell again. Perhaps not for the smallest 95% of web sites, but perhaps for Der Spiegel.