My very-similar-idea of signing PDFs for chain-of-custody (also as a NFT satire) included a "use the cert-transparency chain as a side-chain to solve double-spe

My very-similar-idea of signing PDFs for chain-of-custody (also as a NFT satire) included a "use the cert-transparency chain as a side-chain to solve double-spending" bullet point. It doesn't prevent double-spending (selling the same thing twice), but it can ensure that it can be audited.