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.

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).