You can trade cards with three people in the game. Note: Any other cards can be obtained by killing monsters or buying them from the card lady on Black Rose Street, in Meribia. The first man is at the Spring near Reza and Iluk, and below the western part of Merryod. The second Man is in the Spring all the way at the end in Forgotten Forest (east of the eastern part of Merryod; the city with boats everywhere, not the Forest Of Illusion). He only has one card, and trades it only after you have a sufficient number of cards (which should be sometime near the end of the game, probably after the Grindery event). The third man is in Meribia, near the ship dock. You can only trade with him after the Grindery event.