Just because a game is good doesn't mean people aren't going to sell it. Not everyone can afford to shell out hundreds of dollars each month for all the new games, so they sell the ones they are finished playing knowing full well that they can come back after some time has passed and buy the game back used for close to what they originally sold it for.
Used games would hardly be a market if everyone was just selling trash games. Instead they are the foundation for retail game stores

. Cute soundbite I guess, but nothing based in reality.