To start, the most obvious one and good in the beginning of the season. Start doing the SBC's for easy coins.
Some info about the market.
Price of items are different depending on how many people there are online. Week days between 3 pm and 10 pm, and weekends are times when the most people are playing. (GMT) Prices of consumable cards will get double as expensive.
A time consuming method: Bidding on loads of items at the same time. For example bidding 200 coins on normal gold contracts, you can bid on as many as you want (untill your coins run out). You will win some, depends on time and market how many you will win.
Making smart biddings: This is also a great method of making coins. What do you need? A popular player, with some decent skills who is wanted by many. You need to determine the cheapest buy now price of the player, let's say its 8.000 coins. Now you are going to bid maximum 6.600 coins on every player you find. You will win some, making a profit of 1.000 coins per player.
Smart investments: Value of players fluctuates, most of the time the value slowly drops over time, as more special cards become available. Sometimes the value of a player can peak, when everyone wants to buy him. It's like playing on the stock market, buy alot and wait to see if your gamble was worth it. This can also be done over a short period of time. Price might rise and drop depending on day and hour.
Rich players only: Another thing you can do, later on in the game. Is completely controlling a market of a certain player. (Note: You need lots of coins for this, I tried with 500 000, but didn't succeed). You search for a player that is wanted by many, good quality, popular league and you buy every available card of this player. Automatically raising the price of this player, which you can sell later on.
(This method works well with silver players, since there aren't as many available as gold, and they will go for high prices)
Don't buy packs: 95% of the time, you will lose coins on buying packs. Only once in a while, because it's fun. Good luck on your journey.