Buying a stolen phone will quickly become your worst nightmare. For most carriers, no stolen phone can be reactivated under any circumstance, even if you were completely unaware that you were purchasing an illicit device. That's why Apple created a simple tool that reveals whether or not any iPhone or iPad belongs to someone else.
Head to icloud.com/activationlock and enter the device's IMEI or serial number. For iPhones, find the serial number in Settings > General > About, and scroll down to "serial number." For iPads and iPod Touches, the number is engraved on the back case, near the bottom. The website is, unfortunately, desktop only for now and does not support mobile Safari.
Next time you're considering purchasing a smartphone or tablet on Craigslist, give it a run through Apple's new tool. If you do purchase a stolen iPhone, it will turn into a glorified iPod.