Small addition Can more than one valid address be generated from a private key?
I assume you are talking about different address types?
If so, the answer is yes.
You can derive all P2PKH, P2SH and P2WPKH types from a single private key.
Is knowing some private keys in HD wallets dangerous? Or sell wallet.dat files?
Depends on whether we are talking about hardened (not dangerous; most common) or non-hardened derivation paths.
With non-hardened derivation paths, it is possible to derive the master private key by having access to a single child private key and the master public key.
But since most (all?) wallets are using hardened paths, this isn't an issue anymore.
Selling wallet.dat files? Well.. The buyer has to be stupid to do so.
Both people (buyer and seller) have access to the same seed and therefore the same private keys.
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A private key isn't 32 bits. That would be way too small. It's 256 bits.
It is quite obvious that he meant 32 bytes.