You idea is known as a product comparison engine. When you sign up for an Amazon referral account it's one of the options. It's also similar to the CamelCamelCamel website. You might want to start with https://rainforestapi.com/ they have a hobbyist price tier. Getting access to all the data will be a big challenge, look at the stores and see if they have a product API. Source: over 3 years ago

Mocki is an online tool for creating and hosting mock APIs. It offers easy setup for returning specific responses and supports importing/exporting mock configurations and generating random data. As a cloud-based solution, Mocki facilitates collaboration among team members. - Source: dev.to / almost 2 years ago

Mocki is an online tool for creating and hosting mock APIs. It allows you to set up a mock version of your API and configure it to return specific responses to incoming requests. mocki.io is relatively easy to use and has a wide range of features for mocking and simulating APIs, including support for importing and exporting mock configurations, generating random data for responses, and setting up conditional... - Source: dev.to / over 3 years ago

Mocki - A tool that lets you create mock GraphQL and REST APIs synced to a GitHub repository. Simple REST APIs are free to create and use without signup. - Source: dev.to / about 5 years ago