A funny thing keeps happening to used-car prices, which were supposed to fall as the supply of 3-year-old, off-lease cars and trucks rose.

Off-lease volumes are rising, all right. That was inevitable considering how leasing bounced back following the Great Recession. But on average, wholesale prices for used cars and trucks combined are defying expectations.

That’s important to the entire auto industry and to consumers, because used-car prices have a profound effect on the entire market. Falling used-car prices hurt the value of customer trade-ins and off-lease vehicles. For customers, low values mean they are likely to have to borrow more to buy their next new car. Conversely, higher trade-in values make that next new-car loan more affordable.

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Surprisingly, for wholesale prices of both used cars and used trucks, year-ago comparisons actually increased, for three out of the five months reported so far this year, according to auction firm ADESA. In…