Talk:Gameplay:Achievement System/@comment-26381147-20150531173948/@comment-198.200.85.234-20150531185731

It's because your inventory is literally full. You can't buy items if your inventory has too much (e.g. 100/100 or more). There's a glitch with the exchange hall where it tells you that your inventory is full in Japanese instead of english. Just go to your items in your town and sell some stuff so you have room for the legend stone (note that the legend stones DO NOT STACK in the inventory, so they take an individual slot everytime you buy one).