We understand your situation and we feel sorry for your loss.Bus as shown on screenshots knife was sold for http://prntscr.com/ahuthv 189200 credits.
In your website one cs:go key is sold for 1120 credits.So user got 189200/1120=168.9 worth of keys.One cs:go key costs 2.2 eur in-game so he sold his knife knife
for 2.2*168.9=371.6 eur that is 418.45 usd and its deffinetly not 500$.Currently there is no such knife on market because its value is http://prntscr.com/ahuwbr
from 414 to 756 keys, thats is 910 eur atleast at the moment.This is one of the knife price on your website after crash fix http://prnt.sc/agz9yx 185250.There is only 3,9% diffrence between for how much he sold and you were selling it.
So please make right desicion and undo changes to his account because knifes were traded before ak-safari mesh madness and there was no way to see there was an error in your value system.

Is this a scam or a honest mistake. http://prnt.sc/ahkwix This guy traded his knifes for credits (not even on the day the glitch happened)(He didnt deposit a safari mesh or p2000 fire elemental and he still got blocked from the group) P.S If you refuse to respond, this will only be used as proof to steam support for scamming.