Sins of a Dark Age is a free MOBA that's well integrated with steam (and is only available on steam). You get an item drop almost every match.
Most items can be sold for anywhere between 0.03 - 0.20 € which is not a lot. Some gear recipes can be sold for several euros though. This game's market is not very stable at the moment and it's changing from time to time.
I personally play it because I love the game though, it's my favorite MOBA and I hope it'll get popular soon so earning some money on it is just a plus.
If someone can afford to buy the game he should buy it. Exceptions are bad ports, games with awful DRM that ruins the experience of the player, and games made by greedy companies that put day one DLC's and then ton more just to drain as much money from people as they possibly can, that kind of behavior should be punished by not buying those games.
I also think that it's okay to download games if you don't have money to buy them in the first place.