Would a stolen iPhone 4S from At&t still work with a different sim?

My cousin wants to get an iPhone 4S from some guy on Craigslist. The phone looks to be in perfect condition, but the guy doesn't know any information about it. Plus, he says it's brand new, but he doesn't have the box or accessories the phone comes with.So, it seems a little "fishy" if he would make the purchase and use a different SIM would it work? If it does work, is there any way he can get in trouble by having it or can it be tracked?

Yes.

I knew a guy that "found" an iphone and he still uses it.

It can be tracked

Yes it will work because at&t is a gsm carrier, meaning you can just put your sim card in and be good to go. Now if it was a cdma iphone(verizon, sprint, and us cellular) and if it was reported stolen, the esn would be bad, meaning you couldnt use it on the carrier it was intended to be used on.

Yes it can be tracked by the cops.

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