How is NFT different than a message signed with a private key by both parties?

Say party one is selling something and providing a reciept with their signed Message/reciept using their own Private Keys on Geth Rpc. The reciept is also signed after party one has signed and that message is signed by party two using their private keys. Now both parties have a mutually signed copy how is this scenario different than a nft. Any one can verify using public address of both parties if the signature match. There is a pretty simple crypto API https://developer.mozilla.org/en-US/docs/Web/API/SubtleCrypto to sign and verify data so why are NFT's so different. I mean the signed message can also be saved as strings smart contract.