A locked phone is linked to a specific carrier and has a software-level lock on the device (often called firmware), which prevents the user from using the phone with another carrier. A fully unlocked phone doesn't have this firmware, and in most cases, all you need to do to get started is insert your SIM card.
Fully unlocked phones can be used with virtually any carrier in the US and internationally. However, before purchasing a device, we encourage you to check the compatibility of your data plan with your carrier to ensure that it will work with the device.
Does Back Market sell unlocked phones?
Yes! Whether a phone is unlocked or locked is listed prominently near the top of its product details page.