Confirm phone is not locked to a network

Without unboxing it, you could ask for a copy of original proof of purchase to find out where it was bought from. If nothing about it is on a proof of purchase, then call the place where iPhone was bought from to find out.

You could try to contact Apple while having the person you are buying the iPhone from next to you and ask to confirm if device is carrier lock; Apple will very likely ask to verify the ownership of the device so that would need to be done by person from whom you want to buy the device from.

Enter that serial number here: Check Your Service and Support Coverage - Apple Support

This site will confirm the following:

• Valid Purchase Date
• Status of Technical Telephone Support
• Status of Repair and Service Coverage
• Whether or not the phone has an active AppleCare+ plan.

However, it doesn't provide any information on, whether or not, the phone is tied to a cellular provider contract. That is, if it is locked to a particular cellular provider. If you have any concerns from making this purchase or that the deal seems "too good to be true," I suggest that you don't buy it and get it directly from Apple or an Apple Authorized Reseller.