The first thing to do is sell pretty much anything you can sell (a lot of your reward cards will be un-tradeable)
Use
https://www.futbin.com/ to look up values and see how they change through the day. The values listed are the lowest "Buy It Now" price. You can usually get players for significantly cheaper by buying on "open bids" because people are lazy and just list players with the default minimum list price and default maximum buy it now because they are lazy.
As an example, lets say you open Kevin Gameiro. The default "list now" that most people use would be 650/10,000 and someone would probably buy him for 1,500-2,000 coins so that they can flip him for profit. Based on the most recent market reading from 49 minutes ago, his lowest Buy Now was 2,400 and all of the lowest 5 were at the same price. I would suggest listing him at 2,300/2,400. If he doesn't sell the first time, lower him to 2,200/2,300 and list again or look him up on Futbin again to see if his price changed.
The market fluctuates through the day because of timezone differences between us and
@hayvis people. You can see the trends, but typically the best time to buy players is when
@hayvis is asleep and the best time to sell players is when
@hayvis is awake. So once the market has stabilized a bit after release, I often buy some players in the evening and then just list them for 12 hours at 10-15% more than I paid (EA has a 5% tax so remember that!) By listing for 12 hours you're basically counting on a price change to make yours the cheapest at some point during the peak UK hours. If you have enough coins to purchase like 50 players (lets say 50k at 1k per player) you can list them overnight and expect most of them to sell for a 10% increase or a 5% net profit. If you can buy them for less than the going rate, then even more profit for you!
So for Gameiro, I would expect his price to be at a relative peak now, so we can look at an hourly graph to see what you could have purchased him for last night (I believe times are listed in GMT, which is close enough to Hayvis time). His low point was 0300 GMT which is about 9pm my time. I usually buy between 8-10pm my time, which is 10pm-midnight eastern! Weekdays and Weekends also have slightly different trends. There are times when the swing is like 20-30% between timezones, but I find using 15% listing change for lower priced players and a 10% listing change for higher priced players results in the most success because if you list them overnight and they don't sell, you'll be headed back into the daily trough and there is always a chance that something changes in the market the next day and you won't be able to make a profit at all. Pigs get fed, hogs get slaughtered sorta thing.
So lets pretend you bought 10 Gameiros last night for 1800 coins (you can almost always get players on open bid for less than the lowest BIN price). Based on my theory of listing for 15% more than the lowest BIN in the evening, you would have listed him at 2100/2200 and he would have become the cheapest listing between 8am and 9am Hayvis time and they all would have been purchased for 2200.
Price Paid = 10*1800 = 18,000
Sold for = 10*2200 = 22,000
Tax = 22,000 *.05 = 1,100
Net Profit = 2,900 coins