You can sell players you no longer need to earn some extra money. To do this, you should put them on the transferlist by going to the overviewpage of the player and follow the instructions after clicking on "put on transferlist".
A player stays for 24 hours on the transferlist. During this period everyone can bid on your player. If at least one team offers to pay the requested sum your player will be sold automatically on the transfer deadline.
It's also possible to bid below the asking price. If a team makes such a bid, you will get to choose whether you want to sell your player or not. Bot-teams often do this when no other teams bid for your player. You will have 24 hours to confirm the transfer, if you don't do this, the offer is automatically declined by one of your assistants.
Not all players can be sold. If a player is "unsellable" his market value is 0.
You can't sell players with less than one star in value. These players are not likely to benefit any team. If these players are under 24 you can still try to loan them to another team.
The initial price you can ask for your players depends on the market value of your player. You always needs to ask at least 33% of the market value, but the procentual maximum depends on the value of your player. It ranges from 125% of the market value for cheap players up to 250% for players with a market value above 1 million US dollar.
If no one bids for your player you get the option to automatically try again with a lower asking price.
If no one bids for your player you can also chose to sell your player to the bank. The bank will pay 80% of the market value of your player, to a maximum of 25.000.000 RSD. Players who are sold to the bank are removed from the game.
To do this you should put your player on the transfer list and check the option to sell to the bank when the deadline expires and no bids are made.