Endimion, the second hand games market is only big in the US. It's growing in Europe but nowhere near the size it is in the US...BUT, it's completely outlawed in Japan. Granted this doesn't suggest it had no impact on software sales but it should be taken into consideration.
Also, why is it that the second hand PS2 games can be factored into this equation but not GC or Xbox? Did no one trade those games in? Wouldn't the same % of console owners trade in games and buy second hand regardless of console? That being the case, the ratio still doesn't bode well for PS2's best selling games. I'll grant it has likely a proper ratio for total million sellers (5 or 6 to 1) but as a ratio of best sellers, they really missed a lot of gamers by total install base.