Their advertising wasn't particularly targeted, either. If you typed "non-existent golden unicorn dung", they'd just put up an ad that said "find non-existent golden unicorn dung on Ebay". Anyone who looked at it would know that it wasn't pointing to a particular item and that is was just siphoning off traffic in hopes that you might find or sell something. This is pretty much one of the stupider ways to use Google's tech. It's not surprising that their ROI was dismal.
Not sure why this got downvoted because it's true and we've all seen it. I won't trust anything ebay says about marketing because they are completely out of touch with their own users.
I'd think that it has more to do with the nature of ebay's business. Sure, I might click the link and check the prices on ebay, but why would I want to deal with private merchants, slow shipping, paypal, signing up for an account (if new user) and a bunch of other things that go along with ebay if I don't absolutely have to?
I'd be interested to see Amazon's return on advertising.
I'm utterly confused by ebay's move here. They have the brand exposure. If someone is looking for something and would find ebay acceptable, they would go to ebay directly. Someone who finds ebay unacceptable would not touch an ebay ad. Why would the company think ads would help at this stage?
Not saying I think they're correct, but I would imagine they're targeting people who know about eBay, don't have an opinion regarding them one way or the other, and aren't going to take that extra step to check ebay.com for their next purchase.