To make this glitch successful you'll have to follow these steps:

1 Place any items at the top-left slot (First slot)
2 Get into a random battle
3 Go to Items (Works with any character)
4 Select the Item (While the cursor is highlighting it)
5 Now press UP + A at the same time
6 Press B

Now you should have an item duplicated
Now you can get 99 Items without buying anything!
And having 99 Elixirs is better as well!
Good luck