Re: old Bitcoin addresses as 'NFT' on the Bitcoin blockchain - 'signature chain'
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Now some items on OpenSea: https://opensea.io/collection/signature-chain
The items include the private key of the Bitcoin address for the given 'signature chain' as unlockable content that can only be unlocked and revealed by the new owner of this item.
We are very early.
I think you should warn the users about the risk behind getting this address and private keys, since there are more people who have a copy of those private keys, then the users who buy this will risk their money if they decide to send coins to those addresses.
If the goal is only to use those addresses to sign messages, then there is not a big deal, but people must know how these addresses are a high risk for the funds.