FIFA coins are a virtual currency in the FIFA Ultimate Team and FIFA Mobile with which can be spent for buying packs from the Store, buying items & cards from the Market, and paying for FUT entries.
Earning FIFA Coins
You can earn coins in FIFA Ultimate Team and FIFA Mobile using the following methods:
Selling items on the Market
Selling items Using the Quick Sell Option
Receiving Coins as Gift
Buying and Selling FIFA Coins with Real Money
Buying and selling FIFA coins are considered against EA Sports' Terms of Service, in other words, buying and selling FIFA coins are illegal and by doing this you put your Origin account at the risk of getting banned permanently by EA.