Obviously, the more time your cards are listed, more chances you have to sell them. But most FIFA 19 players are looking through the first few pages only before deciding. You have to make sure your cards are in this first few pages most of the time.
To do this you just have to list them for a duration of 60 minutes, and when the auction expires re-list them for another hour.
There are some exceptions to this rule though.
When you know you won't be able to log-in for several hours, when you go to sleep, for example, you can set a duration of 6 hours for your auctions.
Most of the time, when opening packs you have to be very lucky to get an expensive player.
Actually starting with FIFA 19, EA is displaying pack probabilities and depending on the pack you open and some specific campaign you have a chance bellow 5% or even bellow 1% to make profit.
You can still open a lot of packs though because we all now how fun and addicting it is but all this packs can come from weekely rewards and completing SBCs or FIFA Points for Promo Packs.
Gather your hard earned coins to buy the best team.
Don't discard the players you get in packs even if they are untradable.
If you get an untradable player and you already have it as tradable in your club switch and sell the one that is tradable.
You can use this players in the many Squad Building Challenges we get weekly or sometimes even selling them on the market gets you more coins.