The internet is rife with snake-oil salesmen and pyramid scheme peddlers that will either convince you that they need your bank account number (...and it bears repeating, don't ever do that) or that this evil series of tubes is a den of thieves and liars and all of the easy money has already been made. Even still, if you are a fan of Steam, you've probably participated in a Steam Sale or two, and you've probably heard about Hats, Trading Cards, and various other items that can be sold on their open market between ordinary folks like you and me. If you're an enterprising individual, you've probably pondered whether you might earn a little coin for yourself off this "marketplace" ... earnings that I'm sure you could find a way to spend. Well wonder no more! Below I'll debunk some of the myths and explain some of the tricks to make just a little money, if you've got the time and the patience.
You can't quit your job
Let me start by saying that this article doesn't constitute financial advice, but rather my own experience when buying, selling, and attempting to profit with the Steam Marketplace. Consult your Financial Advisor as applicable, and don't use Mommy and Daddy's credit card without permission :). I also want to clarify that everything I describe in this article is completely "above board" from my perspective - no gray market trading, no "conning of noobs". If that's your thing I suppose you'll have to find your tips elsewhere.
With that out of the way, I must unfortunately share a little more bad news. At least with the ground rules I used, I don't see a feasible way you could quit your day job and solely earn your living from Steam market sales. Profit opportunities are in small volume, the risk profile for trading is often very high, and the time opportunity cost of seeking out profits typically amounts to sub-minimum wage rates. Despite those downers, free money is free money, and over time even small profits can net you the occasional awesome five dollar game.
BUY! BUY! SELL! SELL!
It so happens that there is not any real physical market for Steam where traders in cool jackets yell offers for DOTA Compendium points or scribble notes about Pyro Trading Card futures while they chug Red Bulls... although plenty of money is changing hands. Steam does however provide some nifty tools that give you something of the same effect when you are dealing with items that have been classified as commodities. On Steam (as in real life), commodities are any item that is sufficiently common that large quantities of them are traded and purchasers are primarily motivated by price (since the items are identical).
Be sure that whenever you are selling a commodity, you do so from the Commodity interface (as shown in the picture). This will give you the most information about existing offers, volume, and trading history. Merely going by the "suggested list price" that is shown in the Inventory interface and some other places can cost you some significant profit (but be aware that a lot of sellers use this as their only guidance).
Pro Tip Don't believe everything Steam is telling you with all of its whiz-bang graphics and stats. Trades are happening in extremely high volume, especially during sales, and those stats will often not update fast enough for you to benefit from changing trends. If you need to sell something fast, the Steam suggest prices is likely to move it, but if you can afford to wait (and you almost always can), then look closely at the existing cheapest competitive offer and consider pricing higher than that. Chances are, the natural variation in price will cause your item to sell before long, sometimes almost instantly. For most commodities this won't net you much, but a few pennies will add up towards cheap games over time.
Can you really even LIVE without these hats?
To horde or not to horde
At this point in the maturity of the Steam Marketplace, it is possible to earn a ton of items for doing literally nothing. Trading cards are earned simply by spending time with a game window open, and many in game items like TF2 hats are earned the same way. As the free virtual shiny objects begin to crowd your inventory, you'll need to decide whether you want to cash them in, or stack them high and covetously protect them. For my own part I tend to keep them for games I like, so that I can craft badges and get other virtual schwag for that game - but I cash them in for the games which I'm not as invested in.
Pro Tip When considering to sell Trading Cards, take a minute to consider the opportunity cost of holding on to them. For very popular or free games like TF2, the cards trade near the basement price of around $0.07, limited by the transaction prices of Valve to net you $0.01. Cards for these types of games are worth very little, but when combined into badges during sales, they will net you additional loot which has a chance of being worth more. This means it is sometimes more profitable to buy or better yet trade to get a few cards to complete a set before a sale, and then craft the badge during the sale.