According to a recent change in the Diamond Trade page on the Runes of Magic website, Frogster is now limiting the things you can do with "Diamonds" (Frogster's in-game Cash Shop currency) if you've purchased these diamonds via potentially fraudulent methods. Apparently this change comes about due to the high volume of hackers who have been stealing Credit Card information and PayPal accounts in order to make quick diamond purchases online, only to gift (or sell) these diamonds to their main accounts.
As a way of rectifying this issue, the Runes of Magic team has declared that any Diamond purchases made via PayPal, Credit Card or PlaySpan will be completely unable to trade or gift any of their purchased Diamonds. Instead, Diamonds acquired via these methods can only be used to purchase in-game items for the user, but they cannot be traded or gifted away. Other forms of payment that require purchasing before being enabled, like Runes of Magic Gift Cards, Zeevex Cards and Ultimate Game Cards, will still allow the player to trade and gift their Diamonds normally.
You can read all about the change and what methods will allow you to trade online at the Diamond Trade information page here.