Wilburforce - 11:36 am on Nov 1, 2012 (gmt 0)
They have lists of "buy intent" keywords and key-phrases put together by you guessed it, adwords advertisers.
Yes, they can filter results based on search term (I didn't think that realisation was new to any of us here). I don't see any particular cause for concern if results for a "buy" term favour sellers, and in terms of relevance I don't see why big advertisers (showing a clear strong intention to sell) or OEMs and big established brand retailers shouldn't return good results. Apart from anything else, these are probably not inflating the importance of their services by artificial means.
If Amazon - higher than your site in the SERPS - provides the product I am seeking at a good price, with reliable low-cost delivery, the question to address is: what is it about your site that is more relevant to me?
Looking for the answer in evil profit-led consipracies will have no effect on your ranking.