Building a business off of another’s business is risky. There is a non zero chance that the original business will take steps to manipulate, change, alter, or outright control the outcome. That’s what Valve did.
Tomorrow Valve could decide that the value of crates is too high so they drop the price of crates to a penny a piece. What would that do to this 3rd party market? Poof.
I doubt they will touch crates. There are hundreds to hundreds of thousands of transactions on each type each day. And they take their cut on each of those.
One example I checked was about 0.20 going to Valve on each sold on market. And they sold 280 thousand of them in last 24 hours. So 56 thousand in single day by minimal effort.
Tomorrow Valve could decide that the value of crates is too high so they drop the price of crates to a penny a piece. What would that do to this 3rd party market? Poof.