Need your thinking on how does these Comparision Sites actually work.

Sites like Junglee.com policybazaar.com and there are many like these which provides comaprision of products , fares etc. grabbed from different websites.

I had read a little about it and what i found is-:

• These sites uses Feeds of the sites data.
• These sites uses APIs of the sites which are actually provided by those sites.
• And for some sites which do not have any of these two posibility then the Comparision sites uses web-crawler to crawl their data.

This is what i have found out. If you think there is more things to it please do give your own views.

But i want to know these for my learning purpose and a little for curiosity-

• how does they actually matches the crawled data , feeds, and other so that there is no duplicacy.
• What is the process or algorithms for it.
• And where should i go to learn these concepts. References for books , articles or anything else.