Record and take pictures of everything you are selling.
Document it was working, get the imei on video etc.. safeguard your product so when he turns around and says "IT"S NOT WORKING" etc
record how you packaged the iphone.
This doesn't work nor do video recordings of you opening the package (like Paulson did). Anyone can say that you could have staged either one (i.e. you packed the item with a phonebook, turned on your camera, and THEN recorded opening the package).
Really the only way video recording that would be believable is if you record the entire process continuously, including packing it and then dropping it off and paying for postage at the PO where it literally leaves your hands. The same goes for when you receive it from the P.O. where it touches your hands so nobody can say it's staged.
How many people are smart AND paranoid enough to do this? I'd guess 1% or less. As tech becomes more ubiquitous, then maybe it will become more prevalent. Scammers are betting you're in the 99% who aren't smart/paranoid enough to do it. For large purchase packages, I would suggest that everyone request a pickup from the location so it's received hand to hand (drop off won't suffice as evidence).