The Swamp of Sadness
Unlike most of the previous DQs, 6 doesnt really have any dedicated enemies for gold farming that im aware of. But there's always the casinos.
The most conversion efficient items to trade for cash are Silk Tuxedos, which cost 500 tokens each and sell for 2625. Redeeming all that one at a time takes too long though, so I went for Falcon Blades instead (10000 tokens, sell for 18750 gold). I kept 150k for one Metal King Armor, though.