This. Can't be said enough. There's almost certainly not a concerted effort in Google to de-rank eBay, and you'd immediately hear it leak from one of the many people who would have to be involved if there were.
It doesn't have to specifically target eBay to have the effect one wants. One just has to go after characteristics of spamminess that they didn't prioritize before. Like i said in another post, eBay has been spammy for a while, but it was not penalized.
Think this was one of those cases where doing what's right by user also happened to be a crafty business move. They might have had it coming a while and decided to time it right.
eBay and Amazon are huge spenders on Adwords according to this infochart (which may or may not be reliable): http://i.i.cbsi.com/cnwk.1d/i/tim/2012/01/24/google-earnings...