Absolutely no reason not to buy a second hand one (taking the usual precautions, obviously). However, I have been monitoring the prices on Yahoo JP pretty closely the last two months and, as Roy alluded too, the prices they fetch -- even in Japan -- is very close if not higher (by the time you include auction fees, agent fees and shipping) than to get a new one from Nengun/Greenline (~$1200 to your door with hand controller).
If anyone as a good deal on a used PFC (+ hand controller) for a R32 GT-R, let me know
Lucien.