If your considering purchasing an iPhone 4S off Craigslist, then your best course of action would be meeting at an Apple store, or the carrier it's locked to.
They can check to see if the ESN is good, and make sure the device is operational. Honestly the Apple store would be the very best option, then the wireless carrier the second best option.
If the phones legit, the seller shouldn't mind one bit. It's a lot of money to drop on a device and a good seller would realize this.
That's the best advice/input I can give to you. Good luck and I hope the transaction goes well if you do decide to purchase the device.
BTW:
I've fixed iPhones and offered to meet at the Apple store to prove the device works. The techs at the Apple store have never once minded me doing this. As long as I've made an appointment ahead of time.

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