I've heard it suggested that many of the really high-priced items on Amazon or eBay (not just books) are sellers engaging in a kind of arbitrage, either (1) listing items they don't actually own at a price high enough that if someone places an order they can buy it elsewhere for less and have it shipped directly to the customer and then pocket the price difference as profit; or (2) listing items they don't own to inflate their product list and so increase their rating as a seller, which means they set the prices so high that no one (they hope) will actually order anything. Either way, there will frequently be instances when they're unable to fill an order.
-Philip Marshall