Brass key

Brass Key is a free-to-play item on the OSRS Grand Exchange in the other category.

In-game examine text: "Opens a door that leads into a dungeon."

It has a Grand Exchange buy limit of 15 every four hours, a high alchemy value of 1 gp, and item ID 983.

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Brass Key shows an active flip setup with a margin of +156 gp and 1,106 trades over the last 24 hours.

At the buy limit of 15 every four hours, max profit per cycle is about 2.3k gp — high margin with healthy liquidity.