If Linux users don't buy games, I wonder what these royalties tagged "Linux version" I get every month are. Hm. (Primarily Linux even, many people buy the Windows version and get the Linux+Mac versions for free. Buy for one platform, get all.)
Linux users do buy games, but the market is different. Not just smaller, different. Something that sells well on Windows or Mac may not sell as well on Linux, other times it might sell as well (which is really an achievement considering the market is much smaller, but it's really starved for some kinds of games). Depends on the game.
I made a dating simulator with a realistic facial grooming model for linux and got zero sales.