First of all, if you have any valuable players, sell them, you can always buy them back when you're making money.
Also, the World Cup mode is great for money- winning the online tournament gives 30k, and even winning offline on any difficulty (even amateur) will give you 6k as well as the usual fifa coins
you win every match.
So my first piece of advice is to try and build coins in the boring grind (this was my main way, aided by occasional pack luck).
After you have your 'seed money' of 100-200k, go to Futhead, find some Informs or TOTS players you can afford and make note of their prices. If you click their price it will allow you to refresh the price to the absolute latest, as well as show you a list of all of that particular card on the market, and their prices.
This is how I saw the four Mata cards I mentioned. I then logged in on the mobile app and bought two of them while waiting for the other two to sell or expire. At one point, there were no IF Matas listed on the 'Buy It Now' market, when I put my two up and went away.
You can do this with any card, just make sure that it's rare enough that you can set the price (or at least buy all the cheaper ones so you can set the price to match the more expensive listings and still have a chance to sell) and that there will be a demand (it's a lot harder to sell a Russian TOTS than it is a BPL for example).
If there a few versions of your card at the same price as yours, try to differentiate your version- stick a training card on him, or change his position to stand out. Usually when I try to sell a card I'll go 10%-25% over the cheapest version and stick a chem or training card on mine while listing for 12 hours.
I can't really tell you much other than to study the market, be ready to swoop for a bargain and be ready to hold onto a card if you think the price will rise. There are no guarantees and you'll lose fifa 16 coins
on some cards before you're done, but it's thrilling when you can pull off a great deal. It will be slow going at first, but as we've said already, the more you have the easier it is to make more.
The market really is another game mode in itself at times, try to have fun with it!