The seasonally adjusted annualized rate of new-vehicles recently eclipsed a more-than-eight-year high, reaching 17.5 million units in August, according to the latest edition of Guidelines from NADA Used Car Guide.

And one of the offshoots of a stronger new-car market has been a boost in trade-ins, and this impact has trickled down into the auction arena, NADA’s report indicates. Not to mention, hefty gains in the off-rental channels are leading to volume gains at auction, as well.

Jonathan Banks, senior director of vehicle analysis and analytics at NADA, says in the report that more and more of these trade-ins at dealerships are “gradually ending up at auction.”

In fact, Banks points out there was a 2-perecent gain, on average, in the combined auction sales of 2006 through 2010 model-year units in July and August.

“More significantly, 2013–2014 model year volume, comprised largely of off-rental supply, has also been trending up over the past two months,” said Banks, referring to July and August. “Following weeks of declines, volume for the duo shot up by 12 percent on a monthly basis in July and another 14 percent in August.”

Fitch Ratings addressed much of this in its analysis released in September, as well.

“We expect used-car values to further weaken as off-lease and fleet volumes rise and sales of new vehicles continue to be strong,” Fitch analysts said in its report from earlier this month. “Today automakers reported that light vehicles sales rose by 6 percent in the past 12 months and that the seasonally adjusted annualized selling rate reached its highest level since January 2006.”

As the new-vehicle SAAR hit its highest level since 2006 this July, Fitch acknowledged vehicle trade-ins entering the secondary market increased.

“Additionally, incentive spending remains high. Cash, interest rate subvention and longer loan terms providing for lowered payments have fueled sales, making new-vehicle purchases more attractive,” analysts said.

Editor's Note: Staff Writer Nick Zulovich contributed to this report.