SANTA MONICA, Calif. — Edmunds.com estimated that the average price paid in the United States for a used three-year-old vehicle in September 2010 was $18,832, up $1,471, or 8.5 percent, from September 2009.

“Used cars are in great demand and relatively short supply, so their prices are remarkably high,” said Edmunds.com Analyst Joe Spina. “Automakers understand that many traditional new-car buyers are considering used cars, and in response are emphasizing their certified pre-owned car sales programs through which they can earn revenue on used-car sales. This effort will keep used-car prices high since CPO cars sell for a higher price than comparable non-certified used cars, raising the ceiling for the entire used-car market.”

Edmunds.com estimates that 18.5 percent of used cars bought at dealerships are certified pre-owned vehicles, compared to 13.8 percent this time last year.

“The fact that there is high demand for used cars is good news for consumers selling or trading in their cars, but not for buyers,” commented Edmunds.com Senior Consumer Advice Editor Philip Reed. “We recommend that anyone looking at a lightly used car should also research prices and incentives on new cars, because the deals on some new cars make them even less expensive than used cars.“

Edmunds.com lists new vehicles that may be less expensive to buy new compared with used at http://www.edmunds.com/industry-car-news/new-vs-used-car-buying.html.

The following are among the three-year-old vehicles with the most movement in average True Market Value price since September of last year:

The following chart sets forth average three-year-old vehicle True Market Value prices by segment: