Now, the game tries to remedy the “how do you know how much attack power an item has without buying it?” problem by giving you an in-depth item information screen whenever you select an item, but it doesn’t work. I mean, look at the in-depth item information screen. Up, +10? Down -10? What kind of statistics are those? How do they help you determine what item to purchase? I have absolutely no idea how it works.

I hope you have since realized that the up arrow increases the amount of an item you want to buy 10 fold (or the maximum you can afford up to 10). Down arrow brings you down 10. Side to side bring you one item up/down. Etc.

Nothing to do with attack power.