I know in the Adsense TOS you are not allowed to run other contextual ads. I am not currently an EBay affiliate - but for those who currently are here's a quickie: Can Ebay be set to non-contextual, or is it in any way possible to run the two programs concurrently?
I'm not the expert, but I do run some ebay ads. You can set keywords using the editor's kit and get ads for matching auction items. But as far as I know there are no contextual ads ala AdSense.
Ok - I was under the impression that the ebay ads would contextually look for keywords on your page (like adsense does) - and then display relevant matching auctions. It sounds like I am wrong and you have to manually specify keywords then.