I have no idea about this game's fut as I lost my interest in UT after Fifa 15 but my tactic on Fifa 15 was trading between 3-5am or 12-2pm then sell on Friday and Saturday nights, made an ungodly amount of profit from it. On Fifa 15 my highest amount of coins at the same time reached 9.3 million before I sold it all to people in my school.
Fut 14 was the easiest to trade with and I ended buying all players above 89 at least once to try them out (even the special 99 Messi). Fifa 14 trading was way too easy and I never needed to buy coins. In Fifa 14 my highest amount of coins was around 23 million. On this Fifa it seems like trading is easy too but I don't have an interest as there are way too many coin sellers and too much aids in the gamemode, I stuck with Career mode as it's a better FUT in some aspects
Definitely trading. To me, the gameplay of FUT is really fun (despite some of its shortcomings), but the real addiction is in trading. There is something so satisfying about finding a deal, buying it, and flipping it for an instant profit. I don't play the stock market in real life, but the transfer market is a great substitute. This is the first FIFA that I've ever played, and I bought it in January. I made my first mil by May.
It's all about educating yourself on how the market works, and finding your niche, like one guy said. I did buy the holiday version, so it came with Fifa 16 account, so that helped, but it's all about committing a few minutes per day--maybe 10-30-- on just trading. You'll see your coin total creep up slowly. Once you're over 200k, you start trading IFs, and those profits start to become much more noticeable.
I don't have any squads worth over a million coins, but I have a couple that are worth more than a 100k. Honestly, I find trading boring and I've earned maybe 50k trading in the month that I did it, but it is really easy to get coins by just playing. I have 975,000 in match coins since I've been playing since the game came out, FIFA is the only game I play (stopped playing NHL when I switched to PS4), and I play around five games per day and you get great bonuses for finishing your season. It also helps to stay in the top divisions as your bonuses are higher.