Because everyone else buying these cards has logical heads on their shoulders. The second hand cards are ex mining cards which have been worked to death, and can call anytime.6400 is one of the worst cards ever, and A380 get's smashed in most benchmarks. I really would like to know who would even consider these POS when you could buy far more powerful second hand cards for same money. I wouldn't even take one if you gave it to me for free.
Why would anyone same buy a card from unknown source of unprovable pedigree when you can get an equal card new with warranty.
And this is assuming that a second hand cards market exists in your country.