They add demonstrable value to the first buyer - he got $20 back, after having gotten all the enjoyment he received from the game! And to the second buyer, who

They add demonstrable value to the first buyer - he got $20 back, after having gotten all the enjoyment he received from the game! And to the second buyer, who would likely get $10 back. They're more efficient than Craigslist or Kijiji, so they're able to siphon off more value to themselves, but they're popular because they provide lots of value to individual purchasers.