Yesterday I put in at least an hour of trading and I jumped from 18k to 130k. A lot of it is just buying one player in bulk, then relisting him when his price goes up in bulk. You make easy coin that way. For example, when Lukaku decided to go beast mode against Newcastle, his card price nearly doubled. When Lukaku scored his first goal, I went and bought about 20 of his cards for about 8-9k then his price went up to 12-14k. Then when Lukaku scored his 2nd goal, the priced was up to around 18-20K so I listed all mine at that time for 18k. Made nearly 180k within one football match.
But then there are times when it doesnt work out for you. I bought 15 Welbecks yesterday once he scored hoping he would score again or at least have a good match, but that didn't happen. I am pretty much make 100-200 coin per Welbeck for that.
That's a great tip, I'll be trying that one weekend if I get time, thanks.