Some CS, but TF2 is my main focus. Items are less liquid and the cash trading scene isn't as established. For example, my average margin on TF2 items is 6x higher than CS (~25% vs. ~4-5%). I'm able to buy for less & sell for more than I could in CS.

Knowledge about the game isn't necessary at all - it's just the market itself. Understanding what people are interested in, what items can sell at, how long they take, etc.