Has to be the Rivers/Donaldson JRVGs, though I only ever saw one copy of each for sale while I lived in Tokyo, and they were not sealed. They were also for sale for 4000 yen each. Then again, these didn't come with OBIs or inner liners, so I can't really be sure if they were in fact 'used'.
Regarding the fact that there is only a reprint of the first 20 or so JRVGs, you have to remember that while there are a lot of jazz fans in Japan, a large number of the original 250 JRVGs are still sitting on store shelves there, as either new or used stock. With another hundred JRVGs set to come out over the next year, I can't imagine how a lot of the stores I used to frequent would have made shelf space for 350 JRVGs released during the same period.