Buy high value cards
The bigger the price tag, the better (if you can turn in a profit). Buying a card for 10,000 and selling it for 11,000 gives you a profit of 450 coins. If you buy a card for 1,000 and sell it for 1,100, you only get a profit of 45 coins and you have to go through 10 similar transactions to get to 450. So buy high value cards for the easy money!
Chemistry style cards
Once you have more FIFA 15 coins to invest, buying and selling cards with chemistry style can bring you a ton of profit. For example, go for the Hun (hunter) cards which are the most popular right now. The idea is to know the prices on the market, find and buy a striker with the HUN chemistry style at the price of a regular card, then sell it for more. You will get more profit this way and guaranteed sales.
Become a tycoon
There are a ton of cards in the game, but you can focus on a single one. My strategy was to bid on a popular badge, get a ton of those for at least 10% under the selling price, then sell them for a profit. Bid on all cards that you can find on that price, some of the bids will be successful, then immediately sell them off for the quick profit. In the “mass buy and sale” process you don’t want to wait and keep on the cards, especially the badges or kits, because the market will soon be full of offers and those who are ready to pay nice money for them already have them.
The Monopoly method
This one’s pretty difficult to achieve as you need a ton of coins, but can also give you a ton in profit. The idea is to purchase all the player cards of the same type on the market (say, Messi) and create a need for that player. Then, after a week or so, place them on the market, with the price you want, one by one, so they are instantly bought. You will get rich and hated by most players at the same time.