I've been running contextual ads on this site for a few months, using the CTXbay service that won an eBay developers program award. I don't think I've had many people click through, since the ads aren't very relevant, and seem fixated on the word "Adrian".
Now that eBay has announced its own AdContext service is on the way, I'm planning to replace CTXbay with the official service as soon as I can get access to it. The eBay service has access to a lot more information and is quite customizable. I asked about it at the Developers Conference and was told that I could set a default category and control what kind of items appear.