There is a lot of talk in this thread about writing reviews for Amazon.
But keep in mind the original post is not about writing reviews.
As Sarah pointed out, the original poster was not being asked to write an Amazon.com product review, and the job posting in question does not violate Upwork or Amazon TOS.
The original poster was wondering if he could refuse to compile research using Amazon reviews. That is something that neither Amazon nor Upwork has any rule against. But of course no freelancer is obligated to follow any specific research technique.
Of course the answer to the original poster's question is simple: If you don't want to utilize Amazon.com product reviews as part of your research or validation reports, then explain to the client why you do not do so. Verify whether or not he wants to continue to work with you while you provide the type of research that you ARE accustomed to providing.
I would tell the client that he is welcome to hire other freelancers to do the type of research that utilizes Amazon.com reviews.
Both Amazon.com and Upwork explicitly prohibit writing fake product reviews, which means that both Amazon.com and Upwork are trying to facilitate the quality and legitimacy of Amazon.com reviews. So these companies WANT consumers to be able to trust and use Amazon.com reviews. If Amazon.com did not expect consumers to use their product reviews in making buying decisions, then the company would not invest so heavily in hosting and safeguarding the validity of their review system.
re: "I suppose it is possible to 'compile research' based on false reviews - a dodgy practice."
The original poster agrees.
Amazon.com hopes that people will trust its product reviews, but the OP doesn't believe product reviews on that site are reliable enough for the type of product validation research that he conducts.
The OPs client likely hires writers for affiliate pages, where there is an "article" describing a product or family of products ("The 5 best potato slicers for 2020" or similar) which has affiliate links to Amazon. Rather than buying a number of slicers (or whatever) for the writers the idea would be that the writer finds some positive and maybe one negative point about each product.
Ultimately such articles are kinda fake too if the writer has never clapped eyes on the products.
To me, the only reasonable thing to do about Amazon is to .... (your guess). If you need a good reason to get started, just read about that company in Jessica Bruder's Nomadland.
And yes, being on Upwork makes you aware of the main reason you should never trust reviews. Upwork is a eye opener about reviews, not just on Amazon.
I look at Amazon reviews, but not in the way most people might.
If I'm looking for a tech product, I go straight to the one-star reviews to find the worst complaint someone might have come up with as a result of using the product. If it blew up, caught fire or injured an animal (human or otherwise), I usually move on. You have to read the bad reviews with a grain of salt, however, because some people seem to have outlandish expectations about what things should be able to do. I also search to see if anyone had compatibility problems when using a product with their existing setup.
Then I look at the five-star reviews. What I look for here are the dates of the good reviews. Sometimes companies change manufacturers and suppliers. So if they had a run of great reviews initially, and then things started getting weird, it might be a sign that they changed the factory that assembles the product they're selling or they've changed something in the manufacturing process (maybe they've gone with a cheaper supplier for some component or other), meaning it's not really the same product that got the good reviews in the first place.
I use Amazon reviews all the time. As Renata pointed out, they're a great way to suss out the situation if a product was initially iffy but improved, or vice versa. I find the answered questions most informative because so many product descriptions are vague or incomplete or downright inaccurate in some way.
As for using Amazon at all, the company definitely needs to be held to account for the predatory aspects of its business model. But as someone who lives in a deeply rural area, where buying most things beyond very basic groceries requires driving at least 35 miles each way, I would be well and truly stranded without Amazon.
Phyllis, what I do is I look for the product I need to buy on Amazon. I get the name of one of their sellers and see if they have a site. If they do, I order direct from them.
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