Just remember Swappa fees are $20, PayPal fees are 2.9% plus $.30, so figure $16 roughly, and $10 to ship plus adequate insurance. You're down $46 off the rip.
I agree, the average ATT 128gb plus is down a LOT in the past few days and Swappa is flooded driving prices down.
I too am seeing the average about $525 right now in the sold prices (anyone can list for any amount, they aren't selling at $600+).
So unless you owe significantly less than $460-475 it's not worth it; which would be odd after only 12-13 payments on $850.