This is the vanilla trading behavior: The trade requires dimonds, you give it diamonds. Trading in Minecraft does not stricly require the item given by the trading player to perfectly match the item required by the trade. Instead, the item required by the trade specifies the minimum required item properties, but the passed-in item is also accepted if it has additional data.

There is a setting inside the Shopkeepers plugin config use-strict-item-comparison: When enabled, the plugin will cancel trading attempts if the items don't perfectly match. However, note that any deviation from the vanilla item matching behavior results in client-side glitches. If you enable this setting, players will still see the result item show up in the result slot (Minecraft still assumes that the trade is valid). The Shopkeepers plugin is only able to cancel the trade once the player tries to pick up the result items.

I'm using MMOItems to create special items (vang 24 kara & kim cuong den).