I wanna buy an iPhone 4S off of Craigslist and then setup with an carrier?

But I'm wondering if I buy one will I get in trouble if the person off of craigslist that I bought it from stole it and I go to set it up with an carrier will I get in trouble

Probably not but they'll likely take the phone. If the deal seems to good to be true than it probably is stolen.

To answer your question, if the phone is stolen and you were caught somehow, you can go ahead and say you bought it off someone. If the authorities believe you, the phone would get confiscated. You'd just end up losing the money you paid for the phone and a thief out there would be happily spending the money you gave him.

Chances are, if the phone was stolen, it'd just be the phone without the actual box. If this is the case, I wouldn't recommend buying it in the first place, especially if you're worried it might be a stolen phone.

If you find an item online that you suspect is stolen property, don't buy it. Report it.