This code sometimes doesn't work but most of the time it does.

1. Go to the item shop.
2. Pick up the item you want to buy.
3. Go to the guy and talk to him until he askes, "Buy it?"
4. Say "Buy" and right after that, save it.

It helps to save when the arrow is on the "Buy" icon. Save the game and exit. When you go back, the item should be in your inventory.