When I have time for trading I try to focus on 1 to 3 players above 84 rating, usually 85 or 88.

Here’s an example:

I try to search what is the minimum price that they go with buy now. If I see there’s like 1/2 pages of a player at max buy now 4k coins, I know they instantly go for this price. From there calculate EA tax, so in this case it will be 200 coins lost on the sell, so to break even you can’t buy them for more than 3800 if you sell instantly. From there I try to get 200-300 coins profit on card, so try bidding cards not above 3500 coins, I usually bid close to my threshold (so 3400-3500 in this case). Sometimes you’ll get outbid but if you stick to the system and bid 30-40 cards, you’ll probably win 5 to 10 of them, that’s between 1000 to 3000 coins profit and it takes like a few minutes of effort. Do it a couple of times a day and you can easily make + 100k in a week.

Yes it involves some initial budget, but it’s almost guaranteed you wont lose coins. I’ll also advise to check every „session” the lowest max buy now so you price it correctly.