Usually, people who engage in recovery of randomly found wallet.dat files, search for other victims to sell them to. If you bought such a file, and you've lost hope of recovery, then maybe you can convince a gullible newbie to buy it.

They are not "known". You simply trust the word of the sellers. There is no way to verify the balance of the ever-recovered wallet, because the private keys and/or addresses are encrypted (if any).