The good news is that Apple’s relatively new Activation Lock feature is deterring criminals from stealing iPhones, iPads, etc. This feature lets you ‘Find’ your device if it gets lost or stolen and lock it or wipe it out. A locked or wiped phone cannot be re-activated by the next person unless it is first unlocked by the owner.
If you’re buying a second hand iPhone, iPad or iPod touch (any recent iOS device), be sure to check it’s Activation Lock status BEFORE you buy it! Simply type in the serial number (printed on the device) into the website below. If it is locked and the seller cannot unlock it for you, this probably means you’re dealing with stolen goods. If it comes up clean, you’ll know that you won’t have a problem with the activation of the device after you buy it.