Now you can add custom item!
This version brings an item storage to store your custom item to let you add it to the shop and player can buy/sell it. The custom items will store in items folder.
Create a custom item:
You just have to create a .yml file inside that folder. The name of the file will be the id of the item. And then you can take a look at the exampleitem.yml to know the struct.
Another simple way is use the command /shop createitem <id> to create a custom item according to the item in your hand.
Add custom item to the shop
You can put the id of the item (name of the file) in the category's config just like the material id.
Or you can use the command /shop addcustomitem <category_id> <custom_item_id> <buy> [sell]
Warning: I haven't tested much so it can contain bugs.