The very fact that a wallet.dat might be offered for sale, or, "help me unlock it and I'll pay you BTCBTCBTC" might in itself be a big neon light that the wallet.dat is either fake, or has been forged.
100% in agreement with Timelord2067... I was thinking about a slightly different defenition that boils down to exactly the same result:
"If you didn't create the wallet.dat yourself, or didn't receive it from the verified creator, it's either fake or immoral to own"