"It's safer to say that [trade-in] volume will increase if AT&T does what it did last year," said Trachsel. "It creates a bigger pool of people who may want to trade in their iPhone 4."
NextWorth and Gazelle wipe incoming iPhones, refurbish the devices, then sell them direct to consumers via eBay and Amazon.com, as well as to resale wholesalers.
Scarsella said there was a relatively narrow window for iPhone owners who want to trade in their older smartphones.
"The best time to trade in is from now until the day the new iPhone goes on sale," said Scarsella. "Although prices may stabilize or even rise then, once the new iPhone is out, prices will quickly drop."
Gazelle is buying 32GB iPhone 4s for just over $200, said Scarsella, but those in what it calls "flawless" condition bring in $284. Sellers can get a quote by using the condition calculator on the company's website , or by downloading a free iPhone app from the App Store.
Sign up for Computerworld eNewsletters.