I cast Number Crunching I on the raw BookMooch data (caution: the full data set is about a gigabyte, and you don't need it to duplicate these numbers). I present some interesting numbers below.
Total number of copies on wishlists: 981,103
Total number of copies in inventory: 414,146
Of those 981,103 desired copies, 27,141 are available. If everyone requested all available texts on their wishlist, there would be 387,005 copies left in inventory and 953,962 on wishlists.
420,938 distinct texts are on wishlists but not in inventory.
3,591 texts are in contention: they're in inventory, but not in quantities big enough to satisfy everybody who wants them. The undisputed champion here is Kafka on the Shore, which is on 175 wishlists but the only copy is owned by a guy in the Czech Republic who will only mail elsewhere in the Czech Republic. Most of the runners-up are owned by people whose accounts seem defunct. If you go down the list a bit you can see books that many people want, but that nobody wants very much.