Merchant

Note: You can conjoin this quest, “Sell an item for more than 1,000 gold” (“Whaddaya sellin’?”) and “Sell an item for more than 500 gold” (“Richer Than Ever”) in one joint sale.

If you grind a hero on the Gathering Quest in the first region, 85 Lesser Potions (60 HP potions) will sell for 2,000~ gold. I found this easiest upon unlocking my fourth hero, and using this quest to grind him whilst I used my stronger three to collect armour/weapon supplies.

You'll need 170 Tuvale Moss and 170 Ashe Root. (2x each per potion). It'll grind your hero up, and you'll also gain a random amount of Forest Crystals (40~ gold/1 crystal) to sell during this time – and the quest costs nothing. The amount needed may seem daunting, however, remember the Gathering quest is only 7 seconds. It’s good to throw some extra XP with no damage onto your heroes. Alternatively, just grind on it here and there and stock up your Lesser Potions until you’ve 85-90. I sold 90 to be on the safe side for 1.2k~ - this sold in 18 minutes for 2034 gold and clicked both the 2,000 and 1,000 gold for me.

Alternatively, there are items in the game later on that add up to 2k~ gold naturally, but this is one of the quickest ways to do it whilst also levelling your heroes up.

Other potions that can be used are:

40HP – 220 Tuvale Moss and 220 Ashe Root - 110 Potions
60HP - 170 Tuvale Moss and 170 Ashe Root – 85 Potions
80HP - 100 Coral Moss and 100 Willow Root - 50 Potions
100HP – 90 Coral Moss and 90 Willow Root - 45 Potions

Doing this and selling both your Potions and Crystals in bulk will allow for quicker gold progression and aid in progress towards the “Have [x] amount of gold” and “Craft [x] Potions”, "Sell [x] Items" and "Sell [x] Potions" quests.