This trick requires three memory cards. Memory cards A and B should have your main game save. Memory card C should just be any game with a different name than your normal game. Put memory card B in slot one and memory card C in slot two. Load your game, buy your desired cards, then save and back out to the main screen. Then, go to "Trade" and trade all the cards to memory card B. Insert memory card A into slot one and trade the cards back from C to A. You have now purchased cards and gotten them for free.