Just got an s7 on Craigslist. Factory unlocked AT&T. Want to factory reset to make sure all the old user's data is gone. Will this mess up the factory unlocked status? Thanks!
Depends on how it was unlocked. If it's factory-unlocked, then it was never locke to begin with, so you're safe. If it was unlocked using a carrier/manufacturer unlock code, you're also safe. If it was unlocked using any other kind of software hack, chances are unlock will be lost upon factory-resetting the device.
Before you reset it, make sure to check the accounts option in your phone's settings and log out any you see! It's a common mistake on most phones now to stay logged in before resetting. If the seller's accounts are still logged in when you reset it, you'll encounter factory reset protection and need his login info to set up the phone again.