As always, use something like camelcamelcamel to do price history comparisons for you before buying anything on Prime Day or Cyber Monday (or any other time for that matter).
e.g. from Canada's Competition Act:
> They prohibit the making, or the permitting of the making, of any materially false or misleading representation, to the public, as to the ordinary selling price of a product, in any form whatever.
http://www.competitionbureau.gc.ca/eic/site/cb-bc.nsf/eng/00...
This has been done for a while and it's extremely effective.
> The following is an example of a price comparison based on a fictitious former price. John Doe is a retailer of Brand X fountain pens, which cost him $5 each. His usual markup is 50 percent over cost; that is, his regular retail price is $7.50. In order subsequently to offer an unusual “bargain”, Doe begins offering Brand X at $10 per pen. He realizes that he will be able to sell no, or very few, pens at this inflated price. But he doesn't care, for he maintains that price for only a few days. Then he “cuts” the price to its usual level—$7.50—and advertises: “Terrific Bargain: X Pens, Were $10, Now Only $7.50!” This is obviously a false claim. The advertised “bargain” is not genuine.
[1]: https://www.ecfr.gov/cgi-bin/text-idx?SID=f8c45f090a32596673...
They extracted a million-dollar penalty and concessions to how prices are displayed from Amazon just last year: https://www.cbc.ca/news/business/amazon-competition-bureau-p...
My startup is a price tracker for ecommerce in SE Asia, particularly Malaysia, and we did a write-up about this "great sale": https://sarahshopper.com/my/what-to-expect-on-singles-day-11...
(TL;DR: just like you said, use a price tracker)