ATT isnt going to blacklist it, at least they are not doing so any time soon.  But the previous owner could.   If you trust the seller and are buying through a legit website like eBay or Swappa, you don’t have to worry about ATT.

 If you unlock it, you will know if it’s paid off.  https://www.att.com/deviceunlock/#/

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Google Alert – “iphone 7”