1.Pithing Needle
~$3.29
The number one selling card is week, it has been very demanding. And I can see it in players' side board most of the time.

2.Celestial Colonnade
~$7.96
Doesn't need to tap when attacking, and you can still get the mana after the attack.

3.Verdant Catacombs
~$10.88
The Black and the Green Magic Players should be desperate with it

4.Malakir Bloodwitch
~$4.05
It is hard for me to think of a card which could destroy the Malakir Bloodwitch

5.Earthquake
~$2.12
This card is a MUST for the Red. Out of stock all the time...

6.Avenger of Zendikar
~$3.64
Although its a 5GG, it could be a key card to win the game.

7.Smother
~$2.05
One of the best Black cards in WWK. Most of creatures could be killed by it.

8.Vampire Nighthawk
~$2.31
The card is always demanding since Zendikar released

9.Amulet of Vigor
~$2.79
The untap ability could be a nightmare.

10.Luminarch Ascension
~$2.44
If you could not destroy this enchantment, you are about to lose the game. I know some of the players building their Deck based on this card. No doubt.