jadebox - 9:30 pm on Mar 15, 2013 (gmt 0)
"The study only covered ads for sites that appear in the SERPS anyway."
The thing is ... the way Google is about the "quality" of keywords, ads, and landing pages, it's quite likely that any AdWords ad for a site will only appear when the site itself is in the search results. So, personally, I don't think it's cost-effective to run ads on the search network.
And, of course, eBay ranks pretty well for most searches anyway, so it's kind of no-brainer to see that they shouldn't be paying for ads in the search results. And eBay doesn't have a brand to push in the same sense as other companies.
I wouldn't claim to be typical or to have run ads enough to draw definitive conclusions from the stats, but the best ROI I've had through AdWords is from running image ads which won't appear on Google's search results pages. I see far fewer visits from them, but they are far more likely to translate into sales.