Why do we shop?
The motivation to buy things is strong, and we have come to expect this in almost any game. From a simple adventure game where we just pick things up off the street that may come in useful later, to full-fledged RPG games where the inventory is everything, this motivation is indispensable. Even First Person Shooter (FPS) games aren't immune to this phenomenon.
It is important to keep all of this in mind and look at the real-world when designing any shopping/inventory system for your RPG game. The more it feels like something that someone would do in real life, the more at home they will feel with it, and the easier they will find to use it.
You should always be asking the following questions of yourself while designing such a system:
How does an item add value?
Does it seem affordable?
Is it going to improve the play?
Is it desirable?
Is it better to know what the player will already have and why?
Is it single-use (consumable) or fixed (durable/non-consumable)?
Will it break or wear...
