Steam cards are fun but it's a long plod before you see any kind of meaningful profit. There are other, far more lucrative ways of making money on Steam. Caveat though, I've never tried them.
So all of the stuff below is just what I've learned from surfing around.
Basically the process is buy gems, convert said gems to a booster pack (of Steam cards) and then sell said booster pack.
You'll need to find conversions where this is cost efficient. For example you don't want to pay USD2 for a pack of 1,000 gems, convert 800 of those gems into a booster pack for The Walking Dead, and then sell said booster pack for USD1.
2. Buy and sell Counterstrike GO, Team Fortress 2, DOTA2 items
While playing any of the games above, you could randomly receive a game item (eg. hats in TF2 are particularly prized). You can also win some items by participating in game challenges. Items include skins for virtual weapons, outfits for characters and keys.
This item from Dota 2 is crazy expensive.
Dealing in these items can be quite lucrative. The prices on some of these virtual goods are sometimes more than the cost of an indie game and even approach the cost of a top tier game. Further, there are websites which allow you to trade these items for actual cash.
Instances of fraud are high though since these items are so valuable. If you're starting to deal in these goods, I suggest checking out the sites below:
