The supply of npubs is too large to have a price. If the npub is special like  npubqqqqqqqqqqqqqqqqqq.... then I would probably use it myself starting from scratch.

You can sell it to someone if you aggree they hit you in the head with a big hammer 🔨 after the transaction 😄

The selling of #NIP05 identifiers are way more easy to be sold. You just point to buyer's npub and that's it.