In terms of buying and selling, a Player's Game usually refers to the condition of the game. It's when the gameplay is fine or even excellent, but cosmetically is fair, lacking or plain ugly. A person looking for a player's game is less concerned with how the game looks versus how the game plays, or what mods were added (unless they enhance the play somehow).
In other contexts it means that serious players would like the game. For example, "Simpson's Pinball Party is definitely a player's game. Newbies and casual players likely won't get it because it's too complicated for them."