Note: you can bulk this achievement along with the Richer Than Ever (500 gold/1 item) and I’ll buy it at a high price achievement (2,000gold/1 item).
For this achievement, you can simply sell items in bulk with a total cost of 1,000GP or sell an item that costs 1,000GP. I grinded a hero on the Gathering quest within the first region for the XP and then bulk sold Lesser Potions (60HP Potions) for a total of 2k~ which popped this achievement and the 2,000 Gold achievement. This is one of the quickest ways to get the achievement early on in the game. I also found this to be the easiest due to the Crystals you garner that you can sell for more gold and easier progression overall.
For the Lesser Potions, you can sell 85 in bulk for 2000~GP. To create 85 Lesser Potions you will need 170 Tuvale Moss and 170 Ashe Root.
You can see the item requirements for other potions below:
1,000 gold
40HP – 29 Tuvale Moss and 29 Ashe Root - 57 Potions
60HP - 90 Tuvale Moss and 90 Ashe Root - 45 Potions
80HP – 50 Coral Moss and 50 Willow Root - 25 Potions
100HP – 46 Coral Moss and 24 Willow Root – 23 Potions
2,000 gold
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.