The website Ebates catalogs almost all of them. If you shop with their browser extension installed, it'll tell you when cash back or coupons are available. You can also use the extension Wikibuy or Honey, which have Ebates integration built in, and additionally checks in the background if the item you're shopping for is cheaper on a different website.
If you’ve got experience and expertise in a specific area of business, there are hundreds if not thousands of business owners who would love to jump on a phone call and pick your brain. This can be an easy way to make extra money online and to hone your skills to use in your consulting or freelance business. Sites like Clarity.fm let you create a profile where entrepreneurs and business owners can pre-pay to book a phone call with you for a set amount of time. Whether you’re an expert in raising money for startups, building apps, or running a restaurant, you can make extra money selling a few minutes of your time to people in need.
The difference between Amazon and EBay is that EBay (though still filled with new items) is seen as a used marketplace between individual parties, whereas Amazon (which is filled with offers for new and used merchandise from the 3rd parties) is viewed as a Wal-Mart-type superstore. As a consumer, this difference leads me to use Amazon, so it only makes sense to target on my own demographic.