Well, yeah, I wouldn't expect some obscure indy game to come close to something as legendary as Castlevania.
How does the card thing work? I have a few. You mean I can actually get money/Steam credit for them?
Cards will drop for you as you play the game. This is why people in this thread will answer "I don't know, I was idling for cards" when asked how a game is -- sometimes if a game has cards we leave the game running to get all of the drops. (You get x/2 drops rounded up where x is the total # of cards in the set.)
Then using Enhanced Steam (plug, plug...) it's easy to look at your inventory, see what the lowest a card is selling for, and list it on the market. When it sells, you get steam credit added to your steam wallet.
If you buy the cards, you can craft a badge and get custom icons and backgrounds for your steamprofile as well as experience-points. You can have some kind of e-penis to brag about and some nice backgrounds.
The most expensive cards are somehow always the cards from anime-games.
It also gives you coupons (I randomly crafted a badge this week and got a 50% off Escape Goat 2 coupon... which I used and then farmed for cards, which then sold for like $.60 or so, maybe more).
There's also a theory that the higher your steam level, the more likely you are to get rarer drops or foils (which can sell for more).
Plus as you "level up" you get some bonuses for your profile occasionally -- number of widgets you can use, +5 to your max friend count, etc.
Why the hell would people actually buy virtual cards though? I'm not understanding the appeal.
Or does Steam just automatically hand out credit for them?
You don't need to understand the appeal to make money.
It's like any other thing where you can "collect" stuff., really. The main benefits are what I listed above, but then there are people who are really into collecting and thus fuel the market.
http://steamcardexchange.com