Iphone 5s apple Id account isn't deleted. Is there anything I can do?

so I've only had an android and I had no idea about the iPhone when I bought it. I bought an iPhone 5s off of some guy on Craigslist and he said the phone was factory reset. He didn't seem sketchy at all. But I took it in to at&t and the apple Id wasn't deleted and they said there was no way to access the phone until the account was deleted! There has to be something I can do right? I mean what do I do? Just throw the phone away? Lol jk but seriously the guys not writing or calling back which makes me think it's a stolen phone. There has to be some way to reset it so I can use it! Or else I'm just going to go buy a new phone and this iphone is going to sit in my desk for years. What do I do?

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