It's that the trading market is sufficiently thin that even a few hundred units of freshly off-lease vehicles will crash the price.
David Gerard makes a similar point of Bitcoin in Attach of the 50 Foot Blockhain -- that "market capitalisation" of crytpocurrencies is complete nonsense, due to market thinness. Rather, you want to look at total transaction volume. A large player can move the market significantly on a very small number of trades.
Price, it turns out, is far more like pressure than volume. Punch a large hole in something, and it falls rapidly. Squeeze it to a small point, and you can keep it quite high, but on very small total quantity.
Amazing that mark-to-market is still not restored 10 years after its suspension.