I've always put a little dab of toothpaste on my finger along with some water, then gently rubbed the card. Sharpie, aka permanent marker, becomes amazingly not permanent. I try to stay away from areas that don't need cleaning. Sometimes I'll choose to leave Sharpie markings on a key, such as when someone has noted a date on a key. In those cases, when I add the key to the illustrated catalog, I just remove the markings digitally.

Jim