If you buy a unlocked used or refurbished phone, you will save lot of money both on the device as well as monthly plans. You can switch to any network any time without bothering about termination fees. You can also look for deals on the unlimited voice/text/data plans instead of having contract with one carrier for couple of years.
If You are looking for unlocked & used iPhone, You have couple of options to buy them. You can buy refurbished iPhone even in a local GameStop retail store.
Where to buy refurbished used unlocked iPhone 5/4S/4/3GS
Amazon has both refurbished and used iPhones. Most people buy new products on Amazon. But You can buy used electronics for cheap.
You can unlocked used 16GB iPhone 4 starting from $180 for Verizon, $250 for AT&T. Prices vary regularly. Please check the site for latest price. Look at More Buying Choices on the Amazon site for used or refurbished prices.
eBay has user, manufacturer refurbished, seller refurbished iPhones from multiple sellers. Change Carrier to Unlocked & Condition to Used, Refurbished to see relevant results.
3. Gazelle eBay Store
Gazelle eBay Store has few used unlocked iPhones. Gazelle is a popular site to trade in used iPhones. But there are limited selection of used iPhones on the eBay store as Gazelle sells most used phones via Wholesale.
Glyde has both refurbished and used iPhone 5, iPhone 4S, iPhone 4, iPhone 3GS etc. It is like eBay but mostly individual seller list smart phones for sale. You can find phones by condition quality Excellent, Good, Fair etc. You can find AT&T, Verizon, Sprint, T-Mobile iPhone listings. For example, as of this writing, iPhone 5 16GB AT&T is available for $478.
GameStop sells pre owned & refurbished iPhones. You can go a local GameStop store and check out the iPhones before buying. iPhone 4 16GB AT&T version costs $280. Prices change daily. Please check the site for latest price.