When you collect all 25 cards in a deck, you are considered a master of that deck. Woohoo! Once you master a deck, you cannot trade any of the cards away... of course. :p
You can take the master badge from the card list and add it to your trade post as a badge of honor. :D
Sample master badges:
When you master a deck, use the mastery form to let me know!
You'll also be rewarded with:
~ 2 choice cards
~ 3 random cards
~ 1 Fame