CryptoPunks were created before the ERC-721 standard for non fungible tokens existed. For this reason CryptoPunks are only tradable on the CryptoPunks App. Yuga Labs created a wrapper for CryptoPunk holders to wrap their Punks into ERC-721 tokens. Wrapping a CryptoPunk converts it to an ERC-721 enabling you to interact with other marketplaces and defi applications. For more information go to https://721.cryptopunks.app/