It can also be argued that the national gaming "community" was and always is better connected in Japan than over here, so any copy some random owner wishes to sell has a greater chance of making it to the market and in a more competitive environment on top of that. Here people sell Neo Geo carts for $15 at garage sales and flea markets or dump them at pawn shops because they don't know any better, or they just keep them locked up in the garage for not wanting to toss $260 games as they paid for them, but not knowing where to sell, so the number of copies that actually makes its way to the national / on-line market is potentially much smaller than what exists out there. Not to say the above scenario does not occur in Japan but I digress.
What has been said by the others above is certainly true though.