I am about to launch a project that I cannot disclose at the moment. Our core business will be based on secondary sales fees. It is not clear to me if it is something that should be coded into an ERC721 contract or if it is something that has to placed within the NFT's metadata. We shall have our NFTs minted from our website and then they must be compatible at least with the marketplace OpenSea (possibly also Rarible). Has anyone any experience about it?