Seeing as you have no trust at all, I doubt you're going to find anyone who will agree to an escrow deal where funds are released immediately upon proof of delivery.  As you said, what's to stop you from sending them a box of rocks?

Also, the S4s are nearly worthless now due to shipping costs, I doubt anyone wants those, either.  Forcing them into a package deal is just going to turn away potential buyers.

I have no skin in this game, but just a hopefully helpful observation based on my fairly extensive experience with trading hardware on here.