You know, there are many games that enforce this type of reward system.
The ability to share your credits allows for a multitude of reward style systems: Good RP, Puzzle Solving, Helping Solve Quests... all of this player run and capable of being shared quickly. Credits and the opportunities they provide add a great deal of encouragement to players.
The area of abuse, making multiple accounts to do the "Free credits" of voting are minimal damage causers. even this could be mitigated by only allowing so many credits changed over at a time or per day, if the feeling of such a threat was high.
Ultimately, I assume the difficulty truly lies in coding it...