Supply and demand

The dealers adjust their bid prices to reflect the demand for the vehicle. For example: When the Ford Expedition came on the market, banks were over-estimating the lease residuals (what the car would be worth when the lease expired) - consequently, when those 3-year company leases expired, the market was glutted with Expeditions driving the wholesale value well below the figure that Kelly published as the lowest trade-in value. In turn, that became an opportunity for dealers to make an additional profit on the used car, but it was limited because of the HUGE inventory available - savings were usually passed on to any savvy consumer.

All of those cheap cars that got scrapped during the "cash for clunkers" program have driven up the used car prices for anything with a value under $5000. A lot of those functioning commuter cars got sent to the scrap yard.