What does your dealflow look like? Do you usually trade smaller unusuals into larger ones or are you just flipping big-money unusuals? How do you get the deals?
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.
I'm currently making $10k-15k per month; I'm one of the largest sellers on the game's main third-party marketplace (https://marketplace.tf).
Less of a side project and more of a part-time job, since it's ~4-6 hours of work per day.