Bargaining is definitely a must for a shop game :D
But how about, instead of just letting the players type the number, add a small minigame like Potion Craft has? Not a copy of their system of course. As an quick idea, you could add dialogue that lets the player highlight certain aspects of the gem, depending on the interests and profession of the customer. If the player hit, the sell value and mood increases, if they miss, it decreases or the customer looses interest at all