Note: you can bulk this achievement along with the Whaddaya sellin’ (1,000 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 500GP or sell an item that costs 500GP. I grinded a hero on the Gathering quest within the first region for the XP and then bulk sold Minor Potions (40HP Potions) for a total of 510gold which popped the 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 Minor Potions, you can sell 28 in bulk for 504~GP; I personally sold 34 for just over 600GP to ensure that it didn’t accidentally sell for 499/etc. To create 28 you will need 56 Tuvale Moss and 56 Ashe Root. Other numbers for 500GP on potions are below along with the 1,000 and 2,000 gold stats
40HP – 58 Tuvale Moss and 58 Ashe Root - 29 Potions
60HP - 44 Tuvale Moss and 44 Ashe Root - 22 Potions
80HP – 26 Coral Moss and 26 Willow Root - 13 Potions
100HP – 24 Coral Moss and 24 Willow Root - 12 Potions
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
Also: at the end of doing this for all the levels, my Beserker who I used at level 3 was at level 10.
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.
Overall, I found this the quickest route; I sat and calculated all 4 Potions over all 3 achievements and it took me only about two hours and that was with distraction of watching TV and chatting with friends at the same time.