Getting an ROI through your PPC campaigns

I was out searching for something (can't remember now) that showed me one of those generic eBay ads that say things like:

Bad Breath
Looking for Bad Breath?
Find everything at eBay.com
http://www.ebay.com

Stupid obviously.  eBay cannot sell an intangible item like "bad breath", we all know that.  But it got me thinking.  Who is going to click that ad?  Another obvious answer, only people who want to screw eBay, and the 4 weirdos in the world who want to get bad breath.

Man that must turn into a horrible CTR for any eBay campaign.  Does eBay (and Target, Shopping.com, etc) bid on all keywords at a set price or do they pay competitive prices for competitive keywords and dirt cheap per click on phrases that nobody else bids on?

Anyone know how a company as big as eBay or Shopping.com (read: spends millions a month on AdWords) pays for those keywords?  Per click?  I doubt it.

Anyone have the hard truth?