Buying a used iOS device online can potentially be a way of saving hundreds of dollars -- to be safe, though, it's wise to try Apple's relatively unknown Activation Lock Status tool to make sure you're getting something legal and functional.
Many people are selling Apple devices through services like Craigslist and eBay, but given the nature of these websites, it can be difficult to tell if a product is legitimate. Even if is, sellers can sometimes leave Activation Lock on by accident, and the feature must be off before a device can be used by a new owner.
Located under Find My iPhone at the iCloud web portal, the Activation Lock Status tool lets people input a serial number for an iPhone, iPad, or iPod touch. In the case of iPhones and cellular-ready iPads, users can alternately enter an IMEI number. Apple's website has a guide to locating these digits.
Activation Lock is built into iOS 7, 8, and 9, and flips on if a person chooses to use Find My iPhone. The feature is meant to deter theft by making it impossible to reactivate a device without entering the right Apple ID and password. It also prevents anyone but the original owner from erasing data, or disabling Find My iPhone's location tracking.
Asking for an serial number or IMEI is one relatively easy way of making sure a device is clear to go before a purchase. Sellers who refuse to share this information shouldn't be trusted. Ideally, a better way -- if you can meet in person -- is to have the person disable Find My iPhone in front of you.