Navigate to the league tab in the app , select your league, go to Transfers-sell and tap the plus icon next to 'Trade Players' to list a player.
Here, you can set guideline prices and offer them to your competitors.
It's also possible to sell players directly to Kickbase for their current market value. To do this, tap on Kickbase's offer for a player listed on the transfer market and hold down the checkmark. This feature is new since the 24/25 season.