Mercenaries, or mercs are extremely high skilled players with a huge salary that flow from one team to another. Most of them are over-trained national team players. Because of the huge salaries, they are often cheap to buy, but not to maintain.
Mercenaries are hired for short periods of time to win difficult matches; some managers consider mercenaries to be a good tactic for winning the Cup. Many such teams spend a few seasons gathering money just to pay salaries for such monsters. They also usually sell the mercenaries back at smaller prices so they also lose money there. But they will get a cup and bragging rights so they consider themselves lucky.
Some argue that hiring mercenaries is not in the spirit of the game; others argue that people play this game for different reasons and the fame of a cup win, when done within the rules, is one reason.
As national team players, most mercenaries can not be fired, providing an alternative to farming for national communities to keep their better skilled players.