He must have everything and anything that is a Limited Edition or a Collector's Item. He keeps all items in its original plastic wrap and locks them away in his vault, complete with a retina scanner. He never had any toys as a child, so he vowed to dedicate all of his disposable income to buying collectables when he became an adult.
Each mastery level adds another card to the cardpool for defeating Greg. Each of the bronze, silver, and gold mastery levels unlocks a new card, while it takes all three diamond and onyx levels to unlock a single card.