According to this Comscore report, (thanks for the pointer to this Garth) Ebay is the largest SEM marketer on the web with 800+MM exposures and which represented 4.1% of the total (March 2007 data). It makes sense that their decision to pull advertising on Google made Google blink.

In March, paid search activity in the U.S. generated nearly 20 billion total sponsored link exposures. The top ten paid search advertisers, generating 16 percent of all sponsored links, were all retail or comparison shopping sites. eBay.com led with 802 million sponsored link exposures (4.1 percent), followed by Smarter.com with 366 million (1.9 percent), and Shopping.com with 357 million (1.8 percent).

Ebay also owns Shopping.com which is also an SEM Monster.