I'm not sure you guys quite grasp how selling stuff works.
Okay, so you all know about TF2's hats and the community's irrational obsession with them. Well, Valve decided to be dumbasses and released so-called Unusual hats. The hats have particle effects and as such stupid people who should stick to playing WoW will pay flaxloads of money for them. Like, I'm not even joking when I say this, I've heard of hats selling for upwards of $2000. I wish we could sterilize such idiotic people so they can't reproduce, but I think they're probably asexual to begin with, so it's a non-issue. The only way Unusual hats can be acquired, aside from trading, is through random chance by opening crates. Crates drop randomly while you play the game and can only be opened by use of keys, which cost $2.50 each. The chance of getting an Unusual hat is small, and you'll most likely either get a weapon or some other cheap item, so you'd have to buy a ton of keys and acquire a ton of crates to get a chance at an Unusual hat. How these hats' value managed to get so incredibly hyperinflated beyond this is beyond me, and I don't understand how someone can afford to pay said flaxloads of money for a hat (which is an item that doesn't even do anything in the only environment it exists in, let alone in the real world), because it would require you to be either obscenely wealthy or work a job that wouldn't allow you enough free time to even play the game to begin with.
The selling of items doesn't break any of Valve's terms of service. Valve doesn't care if you sell your items, but they don't monitor or control any transactions that take place outside of the game, including the exchange of money, so any such transactions are done at your own risk.