How the EA tax works
Every time a card sells on the transfer market, EA keeps 5% of the price. The buyer pays the full amount, and you get the other 95%, rounded down to a whole coin. Nothing is taken when you buy a card or list it.
So a card that sells for 12,000 pays you 11,400. If you bought it for 10,000, your real profit is 1,400, not 2,000.
Your break-even price
To get your coins back, sell for at least your buy price divided by 0.95, rounded up to the next price the market allows. The calculator does this for you. Anything above that is profit.
More on pricing your trades: EA FC transfer market tax explained and the transfer market trading guide.