There are cards that sell at auction for less than their quick sale value. I buy them up and discard them for a small profit. It takes a lot of time to make it worthwhile, but it turns out that I spend most of my waking hours dicking around on a computer, so it works for me.
While the margins are small, the return is guaranteed so there's no risk. I bid cards up to the highest amount that I can bid while still making a small profit, and try to do the highest volume I can. I spend everything I have on this, and can turn it over in 20-30 mins. and do it all again. The more I do it, the more cards I can buy the next round, and it just keeps expanding. You'll get outbid 20-35% of the time, by people who don't know the value or who genuinely want the card for their team, but if you bid right up to the edge the other people who know the value will leave it alone because bidding another 50 coins results in a loss rather than a profit. When you get outbid, you get the coins back and you just turn it right around on a bid on another card.
I've gone from less than 1000 coins a couple days ago to about 15,000 by doing this.
My team so far is a Spurs-Crew-USA kind of thing, based around Friedel, Marshall, Sandro, Bradley, Cherundolo & Altidore.
My team is cheap, incomplete and not very good yet. I didn't have the good fortune you had in the packs I've opened so far. I got my manager tasks gold pack, and the best card in the damn pack was Benoit Assou-Ekotto. I love me some Disco Benny, but that was a shit pack. I've had to get creative and grind out some coins to make up for my bad luck.