Personally, I have used profits from selling trading cards and unwanted tf2 weapons to buy games, why not rent films?
But where you can earn enough money? The total value of all items I've earned on Steam probably doesn't even sum to 1 Euro...
You aren't idling Bad Rats then, if you just idle the game you'll earn $0.20 cards every so often. There's a really weird underground economy in steam around cards. I don't really understand the market, it seems akin to smokes in the joint. There are farmers (people idling games), scammers (people taking more value than they're getting), there are arbitration scripts always running for the various steam and third party market places.
A lot of the value I think comes from people who want to have a really nice Steam profile. With enough cards and xp you can get a nice background and a high level. I haven't really looked into it but some people take it really serious and are willing to pay for it.
There a lots and lots of markets where the seller almost always loses. Look at art and music.
It's irrelevant if you live in a dorm or your parent's house - you just pass on the cost to whomever is the responsible party. And you finish with a rather bizarre statement about markets where the seller loses. There are markets where a large number of sellers compete for a shrinking amount of market share with an undifferentiated product. But it's irrational for someone to sell something at a loss in the medium to long term, overall (unless they are a monopolist).
There's a 3rd party application that most serious card idlers use, it uses negligible processing power, just pings the SteamWorks API to say you're playing the game. So if you have steam open in the background, I doubt this adds more visible cost than having a pinned tab in chrome.