The Silver Card(シルバーカード) is an item that, when equipped to a character, is able to purchase items and weapons in armories and shops for half the original cost. The selling price would then be equal to the purchase price for that character. There is only one Silver Card available in each game. In Radiant Dawn, this item will disappear after you leave the base.
In Fire Emblem: Shadow Dragon, the Silver Card cannot be used in the Battle Preparations Menu and the Online Shop.
In Fire Emblem: Awakening, the Silver Card sits in your Convoy, and grants its effect. It can be sold, but gives 0 gold when sold. It does not need to be equipped by a unit, and does not disappear from your inventory.
FE3 Book 2: Chapter 10 - Enemy Thief (wait until after turn 10 to kill any enemies and then a Thief reinforcement will come on the right end of the chapter use a ranged fighter to get at him as he appears behind a wall.)