"The phone is less than a month old and it still has one of his labels on his back, I just can't see how the price disparity formed."
It's a secondhand phone, plain and simple. The best you can do is advertise it as being in mint condition, hardly used, etc., but you are facing a buyers' market - people have lots of similar phones to choose from.
In the end, you'll need to face that fact, and sell it for whatever it fetches. Forget about getting near to its new price, that isn't going to happen - the phone was generally reviewed as being over-priced against competitors when new, and that doesn't help.
Phones like the Galaxy S8 and LG G6 make the Sony look a bit dated, and you can get a better phone like the OnePlus 5T for less money.