In the first instance, I keep them! One of my favourite ecquisitions was a copy of a seventies deck that I picked up second-hand, where the paper had been worn so thin by constant use over the decades that it was a struggle to see the images, they had been written-on by their prewvious owners, they were flaking (where they were thick enough to flake) and they have to really, really be handled with care. It';s a goldmine, a divine deck. I can't imagine a deck too worn to use.
When a card is missing - well, as long as I don't know ...
Write to the publisher? Use it for dropping a random card somewhere randomly in public to entertain yourself and to synchronisticaly hit someone else?