For any card you sell on FIFA Ultimate Team™, EA takes a 5% tax. This is done in order to slow down the inflation.
For example you sell a player for 27.500 coins. Although the buyer pays 27.500 coins you will receive only 26.125 coins, the rest of 1375 coins just disappear.
Use this small tool to see how much coin you actually receive when selling any card.
This tool will show you the profit or loss you make from the sale, the number of coins lost with the tax, a maximum buy price suggestion to make some minimum profit of around 300 coins.