The odds of this being a scam are pretty high. Ask to meet the seller *INSIDE* a store for your carrier. If they refuse, don't even bother. Don't meet in a parking lot or any other place except inside a carrier store. The carrier can verify the phone is not stolen, and will work on their network, and that it is in operable condition. Never, ever, buy a phone any other way.
Scam iPhone sales are the number 1 scam on Craig's List. If the price is too good to be true, it is a scam. Your "new old stock" is a red flag to me, and probably a scam. iPhones sell, they do not sit in stock until the model is no longer sold.
I would bet that the seller won't agree to your terms. If they do, get to the place plenty early and wait inside. If they won't come in, or they want you to come out and show them the money before they come in, it is a scam. Do not! go and show them the money. They will take it, possibly harm you, and leave.