Code for Custom Name Item Detector (Shop)

Need to work on a code block. I need a code that checks whether you have an item with a custom name. (For now, assume that the item is Blue Paintball with the custom name of “Knight Eye”). If you have a Blue Paintball with the name “Knight Eye”, it removes the item and gives a diamond. But if you have a Blue Paintball without the name mentioned above, it refuses the exchange.(make sure the code works for multiple items with the same circumstance[specific name])