Unfortunately I do not believe there is a single simple answer for this. By the sounds of it you would need to do the following:
1. Understand the exact structure of the HTML as it resides in each site that you are targetting.
2. Write some code that would download these target pages on demand. This can be much more difficult than what you might think, given that many product pages can be dynamic in nature (not static web pages).
3. Parse the HTML from the target pages and glean the useful information.
My recommendation: attempt to accomplish this using some third party application or web site.
Tim.
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