If you are looking for the All Item Achievement, you will be looking for all the 1800 "nothings" and trading them to the Nothing Man in Jibral (or the Nothing Robo Mecha in Split Worlds) for items; exploring every other chest in the game will pretty much help augment your item collection. However, there are two accessories you must steal from two different bosses or you will not complete your item collection.

Be sure to steal from the Phantom Dragon south of the Glass Spires on disc 3 for one accessory.

Be sure to Loot or Steal from Ultimate Szabo in the Primitive Cube (Cauldron Room) on disc 3 for the second accessory.