Keep moving the slider across until the button changes (I think from "Make Offer" to "Buy Player")... that's when you know you've reached the selling price and can buy the player immediately.
Printable View
You would think this would be more intuitive and seen visually as a defined amount.
You have to have 20/20 eyesight but it is possible (just) to see the shade of grey change colour on the slider bar at the amount of tokens set to buy player.