I assume the buyer's already tried other games to make sure they work and it's not his system? And cleaned his system? I've gotten the white screen on HuCard bootup before when the HuCard or card port was dirty; cleaning both cleared the problem up.
Yes, this has happened to me as well -- a game "freezes" during boot-up.
Yama, I have never had a "broken" HuCard that didn't work (although someone posted here recently about a System Card that seemed corrupted in some manner... but I still have my doubts about that). I highly doubt that the problem is with the HuCard itself, especially if it was in good condition, as you indicated.