EBay last year decided to enter the online ad market with its AdContext program, which lets eBay Affiliates promote auctions on other sites and collect a sales commission. It's not precisely the same as Google's AdSense program, but it's pretty close.
Google and eBay have each been nurturing developer programs. These aren't necessarily directly competitive since developers can participate in both. But the time-consuming nature of software development suggests that programmers will favor one platform over another.
EBay's Skype telephony service isn't yet threatened by Google Talk, but it could be. Given enough promotion, better support on mobile devices, and click-to-call integration with Google search results, Google Talk could obviate the need for a stand-alone voice client like Skype.
And of course, Google has been promoting Google Checkout, Google Product Search, and e-commerce in general as a way to diversify its revenue stream beyond ad trafficking. EBay executives are clearly paying close attention.