The term "marketplace" refers to the Amazon feature where different merchants can offer and sell their products. Amazon provides the infrastructure and mechanisms to enable trade between merchants and customers. Customers can search for products on Amazon, compare different offers and make their purchases.
The term "platform", on the other hand, is a higher-level term that describes the technical foundation on which the marketplace is built and on which various features and services are offered. A platform provides a structured environment on which various interactions and transactions can take place. In relation to Amazon, the platform is the entire technical infrastructure that includes the marketplace, the merchant portal, customer accounts, product catalogs, and other supporting features.