SANTA MONICA, Calif. — 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 by Edmunds.com.

Overall, December’s used-car prices in the United States 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 percent higher in December 2010 than 2009. Also enjoying a kick in resale value were the Cadillac Escalade (+32.5 percent), BMW X5 (+32.2 percent) and Chevy Tahoe (+27.6 percent).

Edmunds.com said it expects used sales to increase at a slightly lower rate than new-vehicle sales in 2011, and that as new-car sales gain steam, used prices will soften slightly. The prices and incentives on some new vehicles could actually cost less than used models.

Edmunds.com also provided year over year and month to month comparisons of used-vehicle values by segment in the chart below.