"how on earth to find so much gold."
When trading, click on a pile of gold, type a large negative number into the split stack pane, and hit enter, then hit the balance icon. If you still do not have enough gold, repeat. You can also do this drag and dropping stacks out of your inventory, if you hold the control key down to bring up the split stack pane. If you don't want to cheat to get the sword, you could save this to a temporary game, to get a screen shot, etc.
