Google just released (over the weekend) in beta a new targeting feature on the GDN; In-Market Buyers.
This cool new feature within Interest Categories allows the advertiser to target visitors whose site habits suggest they are working their way down the sales funnel and are in purchase consideration mode.
Basically this is the blood brother of RTB Display advertising and is a really nice addition to targeting people, not sites, based on site behaviour.
It generally works on a CPA basis and is best to set bid adjustments based on demographics or by specific topic.
For more info see the Google interest categories support page