If you're pretty late in the game: the best way to go about it is to make a new save file. You can have six of them and it took me 1-2 hours to get the achievement by mashing through the tutorial.
Basically, a few quests before you build Nellie's house, the Mayor should be available in Starfall Meadow. Gift him a few items, talk to him, and hangout with him. You'll hit friendship three with him fairly quickly. Then after you build Nellie's house and he assigns you, Adi, and Maple your roles, you should have access to the trending material. This is the most ideal time to get the achievement.
Set up the displays so they're right by the windows (there's a set of them by the door - turn the camera to see them), put out carpenter items made out of the trending material, set the price to max, and then pray he haggles. If he doesn't, return to the main menu and reload. For me, he recommended a price that was 200% the base cost (which can be seen when setting the price), otherwise I assume you could bump it up to a little over 200% and he'll probably give in immediately.
Credit to Karis over on steam for this, figured I would put it here so others didn't have to go searching for an answer like I did.