Citizens has added support for basic built in GUI shops.
/npc shop edit to open the NPC's shop.
You can also edit any named shop by typing
/npc shop edit NAMEHERE.
You will see basic configuration settings including editing the type of shop from buy to sell to command shops, setting the permission required to view the shop, and whether to open the shop on right click.
You probably want to create a shop page. Click on the feather icon to start editing individual pages.
In this screen, you can drop an item onto the page slots to put it into the shop.
Once in the editing item screen, you can set a cost and a result. Note that a result item must be set or else the item will just sit in the shop looking pretty.