Illegal how so? Maybe I'm misinterpreting what he is saying, but isn't he just getting publicly displayed product information from multiple companies, and then comparing it?

This is the sort of site I'm imagining: http://www.logicalincrements.com/

Yes, this is more or less what I am trying to do. I am considering added images as well

I can't speak to the legality of it, but I don't believe that this is immoral in any sense, especially because it sounds that you have to be approved as an affiliate by the company that displays the data you're scraping.