Buy a used iPhone 4 through eBay.com, Craigslist.org, Gazelle.com and Swappa.com. EBay, Craigslist and Swappa.com are online marketplaces through which iPhone 4 owners can list their phones for sale. Gazelle buys used phones and then resells them. You can also buy refurbished phones through carriers.
Gazelle.com is the most regulated service, offering more guarantee of the quality of the device. Additionally, it is unlikely that the phones Gazelle sells have been reported lost or stolen. Swappa.com is also somewhat regulated, as it is specifically designed around buying and selling smartphones. EBay and Craigslist are more open marketplaces, so it is important to be careful when purchasing a phone. Make sure there are photos of the actual device and be wary of deals that seem to good to be true.