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J.D. Power’s data shows the small-car-price trend may have accelerated in the past two months. The average used Civic, for example, rose in value by 2.7 percent from April 1 through May 11, according to data that J.D. Power collects from new-car dealers that also sell used cars. The price of used Ford Focuses rose 1.3 percent during the same time, from $9,838 to $9,963.

After Mr. Haile sold his Civic, he kept getting calls responding to his advertisement. He wonders now whether he should have asked for more.

“I’ve been getting a million calls for the car,” he said. “I felt sorry that I sold it for that price. I should have sold it for a little higher.”