Why would anyone want to buy a bitcoin address?
If I have the private key, I can empty the account before giving it to someone. If I don't, I cannot do anything with it but look at its transaction history.
Edit: And send coins there.
Someone may want to convert old BTC addresses into NFT and sell them for large amounts later, this idea may achieve great profits after 10 years have passed, as these old addresses become of archaeological value as they are old Bitcoin addresses.
In this case, it does not matter whether you own the private key or not, because this address will not be used to send or receive bitcoins, but only as an NFT.