The transfer market
How the market works: every player is owned once, and moves through auctions or direct offers.
Every player in a league exists exactly once. At any moment they belong to a club, or sit on the open market as a free agent, never both, and never neither. A signing doesn't create or destroy anyone; it just moves them from one owner to the next.
Two ways to move a player
Free agents, and any player a club chooses to list, go to auction, where the whole league can bid. A player still under contract who hasn't been listed can only move through a direct offer, negotiated privately between the two clubs that hold and want the player.
The market stays stocked
The system keeps a run of free agents up for auction at all times, so there's always something live to bid on even if nobody has listed a player recently. A club can also put one of its own players up for auction, up to a small per-club cap, rather than waiting for a private offer to land.
Releasing a player
In the off-season, between the last match of one season and the start of the next, you can release a player from the Sell tab on Transfers. The club receives half of his estimated value, and he joins the free agents, where any club can sign him. A release never takes your squad below 18 players or 2 goalkeepers, and a player on the market or in an open offer stays until that is settled.
Next
See Auctions and proxy bids for how bidding works, Direct offers for private deals, and Market value for what a fair price looks like.