Manta ray

Manta Ray is a members-only item on the OSRS Grand Exchange in the other category.

In-game examine text: "A rare catch."

It has a Grand Exchange buy limit of 10,000 every four hours, a high alchemy value of 120 gp, and item ID 391.

View on OSRS Wiki

Manta Ray shows an active flip setup with a margin of +15 gp and 603,392 trades over the last 24 hours.

At the buy limit of 10,000 every four hours, max profit per cycle is about 150.0k gp — high margin with healthy liquidity.