Read on Google Operating System:
A leaked copy of Google’s quality rater guidelines (PDF), used internally to evaluate the quality of search results, reveals some interesting things about Google’s approach to search. “According to the document, which is dated April 2007 and at least looks legitimate, a quality rater has the job to first research and understand a specific search query – say [cell phones] –, to then look at the quality of a website returned for this query,” notes Philipp Lenssen.
There is a whole section regarding spam, and especially what Google names thin affiliates:
A thin affiliate is a page that exists to deliver a visitor to a page on another domain with a different owner. Keywords deliver visitors to the affiliate page, and links on the affiliate page deliver visitors to the second page, which is owned by a real merchant.This is a revenue-sharing situation in which the thin affiliate is paid a commission by the real merchant for any activity generated on the merchant’s site. Usually the activity will be a sales transaction, such as a product purchase or a hotel booking. The thin affiliate site contains text and perhaps images copied from the merchant site. It offers no (or very little) value-added service while earning its commission. The thin affiliate may also earn PPC revenue by providing PPC links on its page.
By this definition, shopping engines could be considered as thin affiliates, therefore as spam by Google!!! However, there is a clear mention that explains that not all affiliates are thin:
If a page offers some value in addition to its links to the merchant, then it is not a thin affiliate. For example, if the affiliate offers price comparison functionality, or displays product reviews, recipes, lyrics, etc., it is not a thin affiliate, and, therefore, not Spam. Some companies that offer price comparisons or other helpful shopping features in addition to the affiliate link are:
The leaked document is almost one year old (date April, 07), but is still very interesting for SEO specialists to better understand the Google algorithms.