This guide was copied from MemoriesOfFinal's guide over on TrueAchievements. All credit goes to them.
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This Achievement takes awhile as it requires a few other Achievements in the process. This guide assumes the player isn't 100% new to the game by this point of the game otherwise it'll get way too long.
Resting Town:
The Blacksmith/Martin
Merchant with the Snail/Tark
Alice
1. Buy out all limited quantity items, even if you already own them.
1a. Some items have more than 1 available. Buy it out.
1b. Some items are unlimited like the Healing Potions and Resting Potions, these don't need to be bought.
Town of the Exiled:
Stanley
1. Buy out his first stock
2. Buy out his second stock of Owl gear. Available after finishing Levine's Quest (there's a new mandatory item purchase from Stanley during this questline) and starting Stanley's (Quest: Travel) by meeting Levine + Stanley at the Holy Grounds near the Manta docks.
3. Buy out his third stock after the completing the Stanley (Quest: Travel) [Unlocks the Achievement: Come as You Are]
Scorchwhere:
Ferryman
1. Buy his ticket
Halcyon Veins:
Se
1. Buy out all 4 item stocks with Se Coins. The next stock will show up when the previous one is emptied out.
Cooks:
Alice (Quest: Delicacy)
Emily (Quest: Tasty Trip)
Doro (Quest: Practice)
Mia (Quest: Fresh Scent, located at Sky Palace)
1. Buy/Cook all of those
after all of those are done
Alice (Quest: Happiness) [Unlocks the Achievements: Taste of Happiness and Epicure]
1. Buy that last food item.
That should be everything
My last thing was buying out Se's items. Money for Nothing popped up on the side in-game before the Achievement unlocked.