For new Zebra, i would change when-buy to when-gain to make it not too powerful with gainers. Also, Donald always chooses gain instead of buy if it can work, for simplicity.
Other than that, I like it. Though there is unclarity about what to do when it is bought from the Black Market. Finally, “when the Zebra pile is empty” should be “If the Zebras pile is empty.” The Black Market issue could be fixed by saying “if there are no Zebras in the supply.”