After killing the Legendary Moose in Red Dead Redemption 2, you can use its pelt to craft unique items or sell it to a Trapper for money and to unlock cosmetic gear. Here are the steps to sell the pelt:

1. Leave the pelt on your horse.
2. Go to a Trapper, which can be found in Saint Denis, Strawberry, and other locations.
3. Choose the "Sell" option and select the pelt from your horses inventory.
4. The Trapper will buy the pelt and use it to craft new items.

Its important to note that you cannot damage the pelt of a Legendary Animal, so you can fire as many shots as you want. If you let the animal get away, you can sleep for 72 hours to make it respawn. If youre having trouble selling the pelt, you can try tossing it off a cliff or into a lake, which will still allow the Trapper to get the pelt.