The problem with most comparison shopping is that all the results you find are sponsored by a big company. What if the product you want to find is a niche product, or you want to find some off the wall web site selling the product. All your comparison shopping will be among big web sites.
But there are a few tools that will go out and search the web to get your own comparison shopping list.
These tools save you the time of browsing through many sites, and show you any kind of site which appears to be selling something.
So you might want to use this tool in addition to the major shopping sites.
There is actually only 1 tool that does this for you eShopper. This tool actually gives you 'organic' results to look at as opposed to 'sponsored' results.
eShopper will do your comparison shopping while you do something else. Then you can come back and look at what it has found.