A good analogy for the PPC market place is the stock market. Stock markets are large, liquid auctions of underlying virtualized supply. So by analogy imagine:
Now do you have to go to the website of the NASDAQ to buy some CSCO stock? Do you need to call up the NYSE to buy IBM? Of course not, you can buy stock listed on any of these markets through brokers, electronic exchanges and everything else. The key is that the buying process is pretty standard.
So I think it's crazy to have separate stand-alone interfaces for each ad market.
In the ad world, aggregators and agencies usually do the brokering. Google's genius was in accepting credit cards and creating a self-serve model. However that model doesn't scale if you want to buy ads across 20 properties that all have their own interface.
So you'd think that someone would become an "ECN", like Island, Instinet or Archipelago.
Whoever can create one of those for the online ad market might do OK.