Q: "Is it legal for me to aggregate/summarize that information and say 8 of 20 people thought the blender was 'too loud'?"
Q: "If Amazon's customer reviews are not copied verbatim is it copyright infringement?"
R: The answer depends on how much and how many of the reviews are copied. Only your own copyright attorney can evaluate what you intend to do and provide you with actionable advice.
The above is general information ONLY and is not legal advice, does not form an attorney-client relationship, and should NOT be relied upon to take or refrain from taking any action. I am not your attorney. You should seek the advice of competent counsel before taking any action related to your inquiry.
This answer is for informational purposes only. It is not intended as specific legal advice regarding your question. The answer could be different if all of the facts were known. This answer does not establish an attorney-client relationship.
To add to my colleague's responses, I'd note that no matter what Amazon's site or Best Buy's site says at any given time, those sites aren't static, so anything you attempt to aggregate would necessarily be time sensitive and subject to any change in those site's content.
Thus, reliance on any other sites would be a hazard for you, not because of copyright infringement risk, but because of the likely inaccuracy of your analyses as those sites add reviews.
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The three above answers are all very good. I just want to add that, if you plan to develop a website that does this kind of aggregation, and you are serious about launching it, you should strongly consider consulting a lawyer to help you design your service to be consistent with the various legal regimes governing media. If you are paraphrasing any reviews, that raises copyright issues. If you allow users to add reviews or aggregate their own, that raises DMCA and CDA issues. If your information becomes out of date or incorrect, that raises potential defamation issues. Tread carefully.
Please note: this is not legal advice, should not be relied upon, and I am not your lawyer unless you hire me. Good luck!