I'm no Charlie Sheen in the trade market. Tried buying some contract cards and selling them cheap. Bought 20 x silver contracts for 400 and sold everyone for 550... then invested it all into packs in hope I got a rare player that I could sell for thousands... ended up getting nothing and I ended up losing coins with that gamble.
Trading is so addictive.
Packs are a terrible way to make money I find. When I first started I pinned my hopes on finding an IF or rare decent player to sell with which I could fund my team. It just doesn't work.
I only buy packs now because I can't be bothered changing my side on the off chance I do get someone decent.
I make money best when I build a side to sell. For example, I had an idea of building a Copa America based side. Got halfway through, got bored, sold them all and made a profit. Players are where the cash is - picking up cheap Aguero or Eto'o and so on and selling them.