Buy a used mobile device has advantages and disadvantages. You can make a big deal if you buy a device other than the last in his line, but be careful. You can buy an iPhone 6+ at a fraction of the price, but if it is blocked by iCloud, you can not use more than an iPod.
Before anything you should make sure it is not blocked. This can be done by hiring a verification service by IMEI, which checks the database if the iPhone is Apple Carrier locked (locked by the carrier) or has activated Find my iPhone. If so, you can not be enabled for use with your carrier, unless you have access to iCloud account of the original owner. If you do not take these precautions you can keep an iPhone that was stolen and it will be almost impossible to use, so before you buy, we highly recommend to check the IMEI indicated on the iPhone by dialing * # 06 #. Do not rely IMEI shown in the box, sealed as it may seem, the iPhone may have been changed.
The IMEI verification service also gives you useful information, like where the iPhone was bought, the original operator, if it is warranted by Apple and the expiration date of this warranty.