While this is good press for AppNexus, I have no idea why eBay would participate in this article:

For example, say a man just searched for golf clubs on eBay (which has been testing a system from a company called AppNexus for more than a year). EBay can essentially follow that person’s activities in real time, deciding when and where to show him near-personalized ads for golf clubs throughout the Web.

If eBay finds out that he bought a driver at another site, it can update the ad immediately to start showing him tees, golf balls or a package vacation to St. Andrew’s, Scotland, often called the home of golf. If a woman was shopping, eBay could change the ad’s color or presentation.

This sounds creepy, doesn’t it? Again, it doesn’t sound that bad to me given this industry is what I spend 16 hours a day thinking about (though how the heck do they know the guy bought something on another site? that appears to be a fiction)  – but I know that not everyone would feel the same way. eBay would seem to have everything to lose here. I guess it could be that they’ve already had to speak about it publicly for investors and so the cat is out of the bag, but it seems very odd.