I don't see any option in the Android app, but I'm unfamiliar with that.
On PC, visit your own profile, then click "Equipment" - "Sell Equipment" is right inside.
On Mobile, you may do the same once you have found your profile in a browser.
Anyway, the club must be unequipped - not in the bag.
Let me add that those who have a Stand Bag Plus must be sure to unequip that club from all slots of the bag and - surprise - to unequip the bag itself.