I have 5 different websites and every one is in a different niche. For my 2 websites I created them last year before the summer, did some link building, mostly directories and I got for 2 of them in Google all countries even Uk, Poland, Germany and much more in top 10. After january and february those websites fell back to page 3 and 4 but still ranking good in Poland. My other websites, 1 is my own service blog where I provide different services for computing got for rank 4 on 1st page of Google to a second page. My Niche Directory got blasted away but before january it ruled the niche keywords I was competing for.
I get frequent calls from Google about going to their conferences but I didnt go to any one of them, in December I put on my page my successes about my top ranking sites and keywords they are on top pages. After that post, few days after all my websites I put on that page got blasted away from top 10. A month ago I got a call from them should they deactivate my Google Engange because Im not using it so I said ok, do that.
Another conspiracy theory:
There was a secret politician meeting (not secret anyomore as they publish who is attending and what hotel) together with some greatest big companies including Google director and Amazon directory and am not sure if the head of Ebay was there.
So my thinking of what you say about amazon and ebay dominating is all considered between those biggest companies some agreement. Go figure it out? Try harder