Not everything posted on OpenSea is art. Anyone can make their own NFT now and place it on OpenSea, charging a disproportionate price for it. But we can say for sure that everything that is sold at Sotheby's is a work of art. Last year, Sotheby's has already sold several NFT at auction, worth several million dollars. So Sotheby's experts consider some NFT a work of art. Here is a link to one of these messages: https://twitter.com/Sothebys/status/1402996062474760193