Pathfinder: Kingmaker

You'll need 1,700,000 GP at the end of the game. The fey Merchant is Issili, and she can be found at the house at the edge of time and the camp at the capitol gates. Save your game when you find her. Buy THE BOOK OF INCREDIBLE AND SCANDALOUS PARSIMONY from her for 700,000 GP. Exit the buying menu. Talk to her again and buy The Complete Collection Of Useless Writings for 1,000,000 to unlock the achievement. I would then recommend reloading.