There are between 20 - 30 million inexpensive unlocked iPhones on the used market today and many of them are being sold on Amazon and eBay for $100 - $500. Smart mobile phone shoppers can save lots of money buying a used phone that is unlocked. Buying a cheap iPhone can save you lots of money using a cheaper data plan from T-Mobile or another GSM carrier. We discovered do research that one-third of the iPhones being sold on eBay were unlocked. Unlocking the iPhone is the process of using software to modify the iPhone to work with other GSM carriers if purchased from AT&T or CDMA carriers if you purchased a Verizon phone.