You can buy and sell your players

First thing to note when you sell any player on Ultimate Team 5% tax is removed from the sale. So if you sell a 2000 coin player you get 1900 coins. 100 coins is taken away as Fifa 16 Coins  the 5% tax.

To work out how much the 5% tax will be for a player go into calculator on computer or phone and multiply the price of the sale by 0.05. EG 2000 X 0.05 = 100. Then remove the value from the total.

You can buy and sell your players in Buy Fifa 16 Coins  the trading area or you can go to the store and buy packs. These can be bought with virtual coins you make in the game. You can buy packs using Microsoft points also which of course you can pay for with real money.
http://www.fifa16shop.com/