Usually when NPCs are shop NPCs, pick the second option from the list when you talk to them to open the shop.
When you right click a character, the top left button is to trade .-.
The Tab button is used to switch between two weapons, so you can equip 2 weapons at the same time but only use one at a time.
Make sure to pick the correct skills when you add them, you cannot remake the a character with the same name
Items are deleted if you try to drop them.
That's all I know for now >_>