This guide explains how to obtain the elusive Yellow Orb in the Dragon Quest 3 Remake. While the process is relatively simple, the initial steps can be confusing. The key is understanding the role of Merchantburg, a town initially represented as "???" on the map.
Locating Merchantburg (???)

After obtaining the ship from the King of Portoga (following the Black Pepper quest), Merchantburg becomes accessible. Its quest marker appears in the northeast corner of the world map. Sailing west from the coast will lead you to its easternmost edge on the eastern continent.
Optimal Timing for Visiting Merchantburg
While orb acquisition order is flexible, visiting Merchantburg early (immediately after getting the ship) is recommended. The town requires significant growth time before yielding the Yellow Orb. Starting early allows you to collect other orbs concurrently.
Establishing Merchantburg and the Merchant's Role

Before heading to Merchantburg, hire a new Merchant from PALS in Aliahan. Upon arriving at Merchantburg, enter the single building. An old man will request a Merchant to establish the town. Assign your newly hired Merchant; they will leave your party and start the town, giving it their name (e.g., Christopherburg).
Merchantburg's Growth and Subsequent Visits

Merchantburg progresses through five growth stages. You'll receive notifications to revisit. Each visit shows increased town size, culminating in a large cabaret. After the cabaret visit, be wary of a security guard attempting to overcharge you.
During the fourth visit, you'll notice growing discontent towards the Merchant. This signals the final stages before obtaining the Yellow Orb.
Acquiring the Yellow Orb
On your fifth and final visit (at night), the Merchant will be absent. The townsfolk have revolted, imprisoning the Merchant in a house south of their former home.
Speak to the imprisoned Merchant. They'll reveal the Yellow Orb's location. Return to the Merchant's house; a quest marker will appear behind the sofa. Interact with the ground to find the Yellow Orb.
The Yellow Orb is typically the second-to-last orb obtained. The order of the others is generally: Red Orb (Pirates' Den), Green Orb (Theddon), Silver Orb (Maw of the Necrogond/Necrogond Shrine).