I have a feeling the Soul's games might be a bit tricky to get in a few generations. Maybe Asura's Wrath also. Someone mentioned Catherine, and to be honest I reckon their right! As some people have said, Muramasa, Xenoblade, and The Last Story, might be hard to get a hold of, along with Pandora's Tower.
Someone mentioned Zelda. I reckon unless its a collectors edition, the games will be common enough, but will demand $50.00 and up. Collectors Editions will become gold dust, probably not the rarest, but definitely very hard to get.
Call me stupid, but I reckon the Metal Gear Solid: HD Collection might be hard to come by as well. I had a hard time finding that game. I now have a sealed copy on my shelf, and sealed it shall stay!