If you shop blackfriday for Xmas and such;

Check out the extension camelcamelcamel https://camelcamelcamel.com/

It tracks prices on Amazon (or if you don't want extension, you can paste the link on their site). The extensions makes easy to check as your shopping.

This let's you check if 50% off is really a sale or "we tripled the price last month and are fucking you over"

You can also setup price watches, and get email when something's price drops below whatever you set.

(Someone else mentioned Keepa that does the same thing, I've never heard of it, but a quick look shows it has a subscription for some stuff, no thanks)