I believe that because of the discussion in this thread, it is most likely the OPs account was hacked or otherwise compromised, and that the hard drive being described does not exist. It would also be very cheap to bruteforce the HDD if it is encrypted as the OP describes, and anyone would be crazy to part with the HDD for as little as the OP is asking for.

If the OP was telling the truth about the contents of his HDD, I believe it is most likely the coins would be spread among many addresses, probably from mining using his computer. I would believe the inability to prove/disprove the OPs claim is an additional red flag that should discourage anyone from doing business with him.