OpenSea API
Introduced in 2018 by OpenSea, the biggest NFT marketplace globally, OpenSea API is a collection of developer resources that allow users to build personalized marketplaces utilizing Ethereum smart contracts. Developers are able to initiate auctions in any currency, including their own tokens, and gather analytics using OpenSea.js. An NFT needs to be compliant with ERC721 or ERC1155 to be listed on the marketplace. OpenSea does not charge a fee, but developers must complete an application form to obtain an OpenSea API Key.