How do I create a NPC with buy/sell interaction

I am currently working on a rpg game where I need to have NPC included, that NPC should enable player to buy or sell weapons/items.

Is there any similar tutorial I can refer to ?

There are some tuts on how to create an inventory system which should help.

What I am trying to do is when I communicate with the NPC, a list will come out for selection, then once I purchase it, only then the item will go into my character's inventory..

Enable a layer which has a list of items that can be selected and then add in some buttons for buy/sell and some logic for currency and adding to the item to the character's inventory. Check le tutorials.