Used car sales are rising in a positive sign both for the auto industry and for consumer confidence, but pre-owned sales aren't necessarily being factored in by economists, CNBC reported.

If certified pre-owned vehicle sales are added in, auto sales will surpass 19 million for the year, topping the around 17 million to 18 million in annual sales seen in the three years before the credit crisis in 2008.

Used cars are selling well enough to steal market share from new vehicles, disconcerting economists since pre-owned sales aren't included in U.S. gross domestic product figures.

Data from Wall Street firm Convergex shows that new car sales increased by 6.1 percent to a 16.5 million annual rate last month, but certified used vehicle sales jumped 10 percent. Pre-owned cars are on track to sell a record 2.3 million units this year.

"These vehicles are essentially new-car substitutes for many buyers," said Convergex's Nicholas Colas, as quoted by CNBC. "The fact that this segment is growing faster than new vehicle sales is a promising signal on the state of the U.S. consumer."

Certified pre-owned vehicles are expected to post record sales for the fourth year in a row, the Detroit News reported last week.

AutoTrader.com, citing Automotive News data, says used car sales through September are up 9.9 percent for the year. In 2013, carmakers sold around 2.1 million certified used vehicles in the U.S., which compares with just 500,000 three years earlier.

"The (certified pre-owned) market is large and it's going to get larger," said Tom Webb, chief economist for Manheim, a vehicle auction company affiliated with AutoTrader.com, as quoted by the News.

Online marketplace AutoTrader.com projects that more than 9 million certified used vehicles will flood the car market in the next few years.

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