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.
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).
This feeds into the supply and demand component quite heavily, as you can imagine. All secondary effects follow this initial relationship.