Get players on loan then place them in your youth team. Then go to your youth team select the player and then apporch to sign them it's free and simple!>> it works with messi aswell as long as you loan him in the first season, and there wages are really cheap too.
Also good player are aaron lennon, jamie o'hara, anton ferdinand. Check it out!!!
I don't really know if this works but try it anyway. When you are about to sell a player when it says "accept" save the game and turn it off. Turn it back on and click accept and you should get double the money for the player. I really hope this works for you