The iPhone has always had a robust market for used handsets, with even older models still getting rather good returns for their former owners. Priceonomics has done a study of the used smartphone market and compared the depreciation in resale value for the iPhone versus Android and BlackBerry handsets.
Unsurprisingly, the iPhone holds onto its value much better than competing handsets over the same stretch of time. Priceonomics found that after 18 months an iPhone still garnered an average of 53 percent of its original (unsubsidized) price; in other words, you can expect an iPhone that's a year and a half old to fetch around US$300 on the used market.
In contrast, Android handsets only held onto 42 percent of their value, and BlackBerry handsets were a close third at 41 percent. The study also found that most Android handsets depreciate much faster than the iPhone; a used iPhone 4S could probably be sold at or near full retail value months after you bought it, but according to Priceonomics an Android handset would have lost "hundreds of dollars in value" over the same time.
The study suggests that the iPhone's reputation as a premium brand is part of why it depreciates in value slower than its competitors, but other factors are likely at play as well. One important consideration is that older-model iPhones still enjoy software updates and support from Apple; the iPhone 3GS is quickly approaching its third birthday, but it still runs the latest version of iOS. Contrast that with the situation among Android handsets, where many (if not most) models sold in 2011 will be forever stuck on Android 2.3 or earlier.
The sheer breadth of different Android models is probably hurting the resale market as well. Since 2007 there have only been five major iterations of the iPhone, but the same number of Android handsets can come out in a single week, from multiple manufacturers, and with feature sets and names that are difficult for average consumers to sort through.
With all that in mind, it's no wonder the used market for iPhones is paralleling the used PC market, where the Mac reigns supreme in resale value.