XERTiO Wrote: A lot of key vendors (insert the G2A vs. TinyBuild thing a few years back here) have been criticized of this, but..
Some keys are purchased using stolen credit cards. Even if they do a charge back on the purchase afterwards, the game-key usually won't get disabled.
In a more legal way though, A friend of mine used to farm in-game items in different games and sell them on the Steam marketplace, and then bought popular games as gifts and sold them on G2A, that way he turned "steam wallet-money" into real money.
However, I don't actually know how some vendors get these keys in bulk.