iPhone 5S purchase off of Craigslist?

Hello.
I wasn't searching for an iPhone 5S, I was only searching through electronics. There's a seller who is selling TWO iPhone 5S 64GB for $750 and only in CASH.
The phones themselves are still in the original boxing and they even have the plastic wrapping around them. The are supposedly Factory Unlocked.

However, my fear is that the seller will call whoever and get the phone blacklisted. Is there anything I can do to prevent the seller from saying that the phone is stolen?

I haven't purchased it yet. I asked the seller why they were selling it and they told me that they buy and re-sell phones. He also said that he wasn't sure if they had warranty on them. He refuses to text me the IMEI numbers so I can check their status, but then again, $750 is a good deal for TWO iPhone 5S 64GB.

If I go meet up with them in person, then I CAN check the IMEI myself. They'll still be in the box. Can I call apple then and there and get the phone registered under my name? Several questions, sorry. I'm just eh about this purchase.

http://atlanta.craigslist.org/atl/mob/4340944832.html

Btw, here's the link to the listing.