No, you've read it wrong. $320m of that $355m is the revenue from selling 21 million copies of the game (and doesn't adjust for discounted copies).
The 10% cut of transactions figure ($21m) is based on how much money they'd make if all the items on csgobackpack.net were sold at market value. But the actual fraction of those items which changed hands would be much smaller.
Valve is rolling in money from its cut of transactions but these numbers are misguided.
That's fairly reasonable when thinking about it. The sales for an AAA game is in the hundreds of millions of dollars. A free to play game should be able to earn the same order of magnitude through its shop, or there wouldn't be so many of them.
Know any good writing that introduces the game item market to outsiders? I am curious about the thought process of people who spend money in this way, and their perception of value.
I imagine a taxonomy of motivations, like the one Larry Harris has in _Trading & Exchanges_. Presumably some people get entertainment value (because surely there is a kick in seeing a game's world modified beyond its normal rules of state mutation); some speculate on price trends (hence the price graph on opskins.com; what are time series graphs for if not to spark idle dreams of avarice), and others getting some sort of utility (i.e., money laundering).
Skinning is probably a slight misnomer. The skins are applied to the player's weapons and can be seen by all other players as well as spectators. They are, in effect, a status symbol.
This feeds into the supply and demand component quite heavily, as you can imagine. All secondary effects follow this initial relationship.
Market volume in the low billions is probably not enough for a government to do much serious money laundering. Maybe organized crime, but not a major national government.
Trade volume, or market cap? This is an important distinction. You can't multiply the number of items for sale by the market rate for those items. What you're saying is that people are spending billions on CS:GO items and I don't think that's correct; I think what you mean is that buying all the items at their current market prices would require billions.
It seems the $40,000 one was an outlier, but still $4000+ is common. Total market volume is in the billions.