You can sell players and get Spirits, Club Money, and Medals in return.
You can sell up to 10 players at once.
You will get a Spirit that matches the player's rarity and category.
The amount of Club Money you get depends on the player's rarity and his level.
For players of rarity 5★ you additionally get Medals. (Note that you won't get Medals for players with the categories [Ex], [Li], [Ca], [D], [G], [S], or [B].)
*Note that you don't get Spirits for transferred players.
Moreover, you cannot sell players who are (a) in one of your lineups, (b) in Training Camp, (c) up for transfer in ongoing negotiations.