So you had an issue as well?
I sold all my phones on Swappa before, but I've had good luck with just trading them the past few years. Selling on Swappa is a risk also, because you don't know who you're selling to, or what lie they may tell, and give you a bad rating. I guess this was bound to happen to me sooner or later, with as many phones I purchase and trade in. Never had this issue before.
Like you, I thought nothing of it, but it's strange I had a weird feeling after the FedEx lady asked me what I was shipping. Hate to blame her because she may not have had anything to do with it.
The Samsung rep was really helpful. I guess they have video of them unboxing a empty box, but she said she believed I shipped the phone, and they were going to investigate it.