If you have g-coins, then you should totally buy random boxes
and sell them for a lot of gold. If you're lvl 70 and up and you
rebirth, then sell myth gear and the easiest way to get money for
all lvls is to sell random box key for about 10-15 gold. Big Gold
Stars can also be sold between 30 and 25 gold. Fish don't help that
much unless you have gold carp. I hope that helped.