First go to the “Line-up” section which is located in the "Team" tab. Now, place the player in the reserves or not picked area.
After doing this, select the player that you would like to sell by tapping twice on him.
When you have done this, you will gain access to his individual fact sheet. On his fact sheet you will notice a green box at the bottom of the screen that says “Sell player”.
Upon choosing this option, you will be taken to a page with your financial director that has negotiated a price to sell the player for. All you have to do now is select "Accept" and the player will be sold.