Please add Newegg and other non-Amazon vendors. Newegg has an API and everything. Around 10 - 15 years ago, I used to use a simple site called pricewatch that wasn't too far off from this.
> if you choose to display prices for any Product on your Site in any “comparison” format (including through the use of any price-comparison tool or engine) together with prices for the same or similar products offered through any web site or other means other than an Amazon Site, you must display both the lowest “new” price and, if we provide it to you, the lowest “used” price at which the Product is available on the Amazon Site.
... which implies that it's not forbidden, but I can't blame someone for not wanting to risk it.
[0] https://affiliate-program.amazon.com/help/operating/policies
https://pcpartpicker.com/product/MwW9TW/western-digital-inte...