That's a valid observation, but if getting a 12-pack of identical decks is not a problem, it's enough to build 13 custom decks (all Aces, all 2s, etc).

Edit: if the players can announce only what suit they are buying and selling but do not show cards to the others, it may end up difficult to deduce what cards are missing, because only the buyer and the seller know the value of the traded card.