Get a PICTURE of the phone's IMEI number. Better yet, when you meet the guy, meet him either in a Verizon store or call Verizon in front of him and give them the IMEI number. You can find it under the battery on the back of the phone. Ask them if the phone is valid for activation as you're buying it secondhand. They'll be able to tell you before you purchase it. DO NOT let the seller call for you, call yourself.
Just a note, I don't ever buy devices from Craigslist. I'll buy them from Swappa whenever possible, as they verify this info before they allow a device to be sold. I'd check out Swappa and see if you can find a comperable deal, but if you choose to try to buy this Gnex, ensure it's valid for use with Verizon.