A lot of patience. The main thing you need to remember is buy low, sell high. Check the average value a player is selling for over the course of a day and set yourself a target you will be prepared to pay. I've been using a Ligue 1/French team. Once I realized it would take me a while to get anyone a bit more expensive, I sold my team making a a few hundred on each player profit, then pretty much brought the same players back for less than I originally brought them for, which allowed me to afford some higher end players such as Reme, Sagna, and that Marseille Keeper. After making it to Division 1, that's where i've struggled. Now i've just sold nearly my whole team bar my goalkeeper and a few I haven't been able to shift for 75,000 coins. With that and money I already had i've just brought Benzema for 78,000, just to try him out. I have now put him on the market place starting at 85,000, buy now 90,000. Too high but i won't be playing for a couple of days so all I need to do is keep putting him back on the market with the web app. Way it's looking right now, should be able get rid of him for 82,000 at least.
So yeah, keep buying and selling really and always put a Buy It Now price set at least double profit.
* A good tip is to leave a player selling over night, when the market is less busy. You're more likely for someone to pay a Buy It Now price if they want that specific player instantly.
The opposite goes for buying. Buy players in the evening during the peak hours. More players on the market = decreased value. Easier to hold out 5 minutes for the next player, rather than the next hour at less busy times.
Also Formation. No point trying to make money on someone who's set to an unpopular formation like 343, as most people only search for their formation when looking for players, so try and pick up a formation card cheap (takes time) to at least one of the formations that have 4 at the back.