OpenSea directed CNBC to a blog post by Finzer, in addition to the social media post. In it, the OpenSea CEO adds that classifying NFTs as securities would "misinterpret the law" and that he is confident his company "operates legally" and that its "users aren't trading securities."

So far this year, the SEC has sent Wells notices, filed lawsuits, or reached settlements with a host of crypto firms focused on ethereum and decentralized finance, including ShapeShift, TradeStation and Uniswap. The agency is also reportedly investigating the Ethereum Foundation.