Theres very little guaranteed money to be made by trading.
Most people trying to sell their players are selling them to up their finances a bit rather than creating a huge cash pile. In the one off case that a player get sold for less than 3000 I already have worked on my finances by not having to pay him his wages next week. So I dont care if I m selling a 3.5* or a 5* player, I tend to set the initial price at 1000 rather than their "value"