In other words, the seller somehow had remote control of the laptop and locked it remotely? Things like this are why you should always reformat and reinstall all the software on computers you buy...

Bruteforcing it is the long way, the fastest way would be to use an EEPROM programmer to clear the EEPROM and rewrite the BIOS. This can usually be done in-circuit without any soldering with a little clip that goes onto the chip. A lot of laptop repair shops have this setup. Some of the newer models store the password in TPM, which caused the shops to initially replace the TPM with a new blank one but some RE later revealed that many (not all) can be cleared in the same way as before.

The fact that BIOS/EFI passwords can be easily cleared must be one of the worst-kept secrets in the computer industry - the companies will often officially say that there is no way to reset them and that the whole motherboard has to be replaced in a not-so-successful attempt to propagate the myth that this protection is so secure that even they can't bypass it.