Each site owner need not do it on his own . Perhaps there is an opportunity for an open source ad hosting solution which has all the features and connect to one

Each site owner need not do it on his own . Perhaps there is an opportunity for an open source ad hosting solution which has all the features and connect to one or more central marketplaces where advertisers can place bids and also each ad can have be rated basis user responses and feedback and you can configure which marketplaces to list and restrict the quality of ads and the kind of tracking