DavidZHawk asks, “What if Google Declared War on Comparison Shopping Engines and No One Noticed?” and points to an Inside Adwords blog post (my bolding):
The following types of websites are likely to merit low landing page quality scores and may be difficult to advertise affordably. In addition, it’s important for advertisers of these types of websites to adhere to our landing page quality guidelines regarding unique content.
* eBook sites that show frequent ads
* ‘Get rich quick’ sites
* Comparison shopping sites
* Travel aggregators
* Affiliates that don’t comply with our affiliate guidelines
Comparison shopping sites and travel aggregators are just two classes of the many flavors of vertical search engine, although they monetize better than most because of the high proportion of transactional search queries. As a result they have been able to afford to buy traffic through Seach Engine Marketing (SEM) where other vertical search engines have not been able to afford to due to lower monetization rates.
When you combine this move to send less traffic to vertical search engines with Google’s more aggressive inclusion of “One Box” search results from Froogle and their other owned vertical search efforts, you start to wonder if Google is looking to keep more of its traffic recirculating within its own properties. iGoogle and Gmail were the first signs that Google might aspire to keep control of more of the traffic that starts there.