Customer NPC?

Hello.

In my project, my character has a shop (he will sell fruit and vegetables).

My character buys fruit and vegetables in bulk. These items appears in their own inventory. I have no problem so far.

What I want is for my character to be able to sell the items in the inventory to the NPCs that come to my shop.

I can bring NPCs to my shop with the behavior tree tool. But no after.

For example, my character has two watermelons and two apples in his inventory. Waiting for customers in the shop. After 3 minutes an NPC comes and orders 1 watermelon and 1 apple. I sell orders to him. and after taking his money the NPC leaves the shop. Or I want 1 peach and I reject it because it is not in my inventory and send it again.

Can you give an idea how I can do this?