An estimated 18.6 percent of used cars sold by franchise dealers in 2010 were certified, up slightly from the 17.8 percent reported in 2009, according to sales figures released today by Edmunds.com, the premier online resource for automotive information.

Overall, December’s used car prices in the U.S. were up 8.2 percent over December 2009, even as values decreased slightly compared to November 2010. Edmunds.com reports that the average transaction price in December for a used three-year-old vehicle was $17,954, up $1,368 over the previous year, but a decrease of $257 compared to November.

“The people who benefit most in this used car market are car shoppers with a high-value trade-in,” said Joe Spina, a senior analyst at Edmunds.com. “Strong demand, a continued short supply and an effort by carmakers to boost certified sales are keeping used car prices elevated, so dealers are happy to take in high-value cars on a trade.”

December was a particularly good month for consumers selling Chevy Suburbans. The average retail used car sales price for the SUV was 32.5% higher in December 2010 than 2009. Also enjoying a kick in resale value were the Cadillac Escalade (+32.5%), BMW X5 (+32.2%) and Chevy Tahoe (+27.6%).

In 2011, Edmunds.com expects used sales to increase at a slightly lower rate than new vehicle sales, and that as new car sales gain steam, used prices will soften slightly. Still, Edmunds.com suggests that consumers interested in late model used vehicles should keep an eye on prices and incentives for new vehicles since those new vehicles could actually cost less than used models.

Since used vehicle value increases are not universal, Edmunds.com reminds consumers that they should educate themselves prior to visiting the dealer to ensure they get a fair price for their trade, as well as for the vehicle they intend to purchase. Consumers looking for pricing information on specific vehicles can check the True Market Value® on used cars at Edmunds.com