The first wave of early adopters is still going strong, absorbing limited stock of iPhone 6 and iPhone 6 Plus devices. But while retailer inventories are shrinking and their back orders are growing, the market is still strong for upgrades through wireless carriers and outright sales through e-commerce platforms.

Sprint and T-Mobile, the pair that appear most excited about Wi-Fi calling, are offering to match any competitor's deal for trade-ins on any iPhone -- yes, that includes the 4 series.

Sprint's deal-matching service is part of its popular buy-back program, while T-Mobile's offer is time-sensitive. Both wireless carriers will apply trade-in credits to consumer bills, though T-Mobile and Sprint project a wait of between two and three business cycles before the value is applied to customer accounts.

Verizon and AT&T customers can get a 16GB iPhone 6 by trading in an iPhone 4, 4s, 5 or 5c. Any of the aforementioned trade-ins will net consumers a $200 gift card from Verizon, which could be applied to a 16GB iPhone or any of Apple's other new smartphones.

The iPhone 5s nets a $300 value from Verizon and AT&T, which can be applied to a singular upgrade. All of Verizon and AT&T's trade-in deals end on Sept. 30.

Not everyone is sold on the new iPhone, either because of its hardware or they're waiting to ensure that all of the kinks have been ironed out. Other consumers are just warming up to Apple and the iPhone 5 series is looking really attractive to them.

For individuals looking to jump ship in favor of some Android hardware or are looking to move from a 4 series to an iPhone 5, now is a good time to sell older iPhones on online market places like Amazon and eBay.

The current going rate for past models of the iPhone matches up roughly to what wireless carriers are offering in their trade-in programs, though a nice case or an extra charger could sweeten the deal for buyers and bring in more money for iPhones listed on online marketplaces.

Before putting an old iPhone up for sale, it is always important to remember to back up data and erase the old contents of the phone before handing it over to a stranger. In addition, iPhone sellers should ensure their phones are "unlocked" and able to operate on any carrier's wireless network -- cell service providers can guide users through the process of unlocking any handset.

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