The sale of in-game resources for real-world money is usually disallowed explicitly by a game's terms of service. One notable case was Blizzard's successful injunction against Peons4Hire, a World of Warcraft gold farming company.
Setting aside terms-of-use considerations (which vary by game/platform), the sale of virtual goods for real money is broadly legal. Wikipedia has a short list of jurisdictions with notable decisions on virtual sales; see the page's references for the official news stories or government documents. Most of the rulings have been positive for virtual-goods vendors, though China has banned the sale of real items for virtual currency (the opposite of your proposed plan) and South Korea has banned the sale of virtual goods for real money outright.
Such sales incur normal tax responsibilities, just as any other income does. I don't know if any jurisdictions have imposed special legislation that place income limits or special tax rates for income from the sale of virtual goods.
In the case of Steam specifically, the sale of virtual goods is broadly disallowed in Section 2.G of the Steam Subscriber Agreement, but narrowly allowed in Section 3.D. All virtual Steam goods remain the property of Steam, and you have a license to use them, which includes sale in limited circumstances.
The prohibition in Section 2.G reads:
G. Restrictions on Use of Content and Services
...you are not entitled to: (i) sell, grant a security interest in or transfer reproductions of the Content and Services to other parties in any way, nor to rent, lease or license the Content and Services to others without the prior written consent of Valve, except to the extent expressly permitted elsewhere in this Agreement (including any Subscription Terms or Rules of Use); ...
And the narrow permission to perform trades in the Community Marketplace in Section 3.D (per the above "except to the extent expressly permitted elsewhere in this Agreement" exception of 2.G):
D. Trading and Sales of Subscriptions Between Subscribers
Steam may include one or more features or sites that allow Subscribers to trade, sell or purchase certain types of Subscriptions (for example, license rights to virtual items) with, to or from other Subscribers (“Subscription Marketplaces”). An example of a Subscription Marketplace is the Steam Community Market.
You can use the Steam Community Market to trade items for real money; this kind of real-money sale is explicitly allowed and facilitated by Steam. Separately, you can also trade giftable games (N.B.: game trading does not support real money) using Steam's trading system. Applying terms to a game-trade (like requiring payment external to Stream's trading system) appears to be disallowed by the broad prohibition in 2.G, but it's not very clear. The mechanical transfer of the game from one account to another is allowed, but the fact that it is actually a sale in a way that is not explicitly allowed seems to make it against the terms of service. I have no idea to what degree this is enforceable nor to what degree Steam polices against trades that involve Stream-external money transfers (though a cursory search of conversations in the Steam forums suggests they might not care very much).
