For this achievement you need to get every item in the Box of Secrets shop in Knothole Island, you need to finish the weather quest before doing this otherwise not all the items will be available. Here is the list of those items, to get them you need to trade certain items for them, in this list you can also find the places we�re these can be found and their economy rate.
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Need: Regal Purple Dye Where to get: Traveling Dye Trader Economy Rating: 5* Reward: Brodican Mace
Need: Crucible Peanuts(x2) Where to get: Crucible Food Trader Economy Rating: 4* Reward: Chasm Outfit
Need: Crunchy Chick(x3) Where to get: Food/Fish Trader Economy Rating: Reward: Wretcher�s Blade
Need: Lute(x2) Where to get: General Store Economy Rating: 1* Reward: Axe of Disharmony
Need: Monk Robe(x1) Where to get: Oakfield Cloth Shop Economy Rating: 3* Reward: Judge�s Steel
Need: Murgo�s Big Book of Trading(x2) Where to get: Bowerstone Bookstore Economy Rating: 5* Reward: Brodican Warrior Queen Outfit
Need: Pure Experience Extract(x1) Where to get: Bowerstone Market Economy Rating: 5* Reward: Hal�s Rifle
Need: Diamond(x1) Where to get: Bowerstone Market Jeweler Economy Rating: 5* Reward: Staff of Wrath
Need: Eternal Love Ring(x1) Where to get: Bowerstone Jeweler Market Economy Rating: 5* Reward: Greaser Wig
Need: Marriage and How To Survive It(x1) Where to get: Bowerstone Market Bookstore Economy Rating: 1* Reward: Knothole Skirt
Need: Pretty Necklace(x1) Where to get: Jeweler/ Gift Trader Economy Rating: 3* Reward: Black Wheel Gang Outfit
Need: Puny Carrot(x2) Where to get: Bowerstone Old Town Economy Rating: 2* Reward: Assassin Outfit
Need: Table Wine(x2) Where to get: Bar Economy Rating: 3* Reward: Cascading Hair
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*Note: The economy rating of the items is very important in your quest to find the rarest items. The items the seller sells are equal or one star lower than the region economy, so you have to be aware of that. Simply go to the �Regions� section where you fast travel and the region economy is shown there.