There was one of the biggest mistakes many players made in FIFA 14 apart from buying the game, of course. They sold all their players straight away but the next day they checked the market prices and the player prices soared even more due to the increase in the number of players and the higher demand for cards. So try and hold on on to the rare cards or more expensive players.
Investing in players is a risky method like most of the methods listed above but if it works it pays of handsomely. Investing in gold players is what I think is the most sensible move, but silvers and bronzes is also very effective. Try and invest in players that have a big stand out stat or are well rounded in all stats. Example is Lennon his stand out stat is obviously pace which is quite an op stat in every fucking FIFA. He was selling for ridiculous prices due that stat only becuase all his othe card stats are mostly below par. So I suggest investing in players who have a good pace, shot or dribbling stat.
Suggestions for players you could invest in at the start of FIFA 15 Coins:
Strikers: Ciro Immobile,DANIEL STURRIDGE (recommended) ,Adrian Ramos,Romelu Lukaku, Vincent Aboubakar, Alexandre Lacazette, Divock Origi
Midfielders: James Rodriguez,Ivan Rakitic,PAUL POGBA (recommended), Kevin Mirallas, Memphis Depay, Juan Cuadrado
Defenders: Thiago Silva, Gary Cahill, Serge Aurier (recommended), Luke Shaw, Mattia Di Sciglio, Carvajal, Eliaquim Mangala
Note: Only invest in these players at the start of FIFA 15.