An influx of aging iPhones in the secondary market could be a key indicator of Apple's holiday season success, according to Nik Raman, co-founder and CEO of resale website uSell.com.

To gauge the strength of Apple's holiday iPhone sales, Raman compared the price decline on uSell.com's secondary market for the most highly traded-in iPhones from 2013 and 2014 leading up to Christmas. Those models were the iPhone 4 in 2013 and the iPhone 4s in 2014.

On CNBC's "Fast Money," Raman said the offer price for the iPhone 4s fell by some 50 percent leading up to the 2014 holidays. That drop, he said, indicated a flood of devices into the market, as people looked to unload their older phones after upgrading to newer models like the iPhone 6 and 6 Plus.

Raman compared the drop in resale price of the iPhone 4s in 2014 to that of the iPhone 4 in 2013. According to Raman, iPhone 4 prices only fell by around 20 percent in the secondary market leading up to the 2013 holiday season. "What we found was that the depreciation for the 4s this year is far more steep and dramatic than the 4 was last year," he said.

Based on those numbers, the 2014 holiday season appears to have been strong for new iPhone sales. "We do think that Apple had a great season. We think that it's continuing into January," Raman said.

Raman's findings mesh well with a report published by analytics firm Flurry at the end of December. By tracking total device activations from Dec. 19-25, Flurry concluded that Apple products accounted for more than 51 percent of all new device activations the week up to and including Christmas day.

According to Flurry's report, "…it is safe to say that Apple's newly released iPhone 6 and iPhone 6 Plus have had a blockbuster holiday season…"

—By CNBC's Michael Newberg