This is a jump to the extreme here but wouldn't you want to support the developer who made your game regardless whether you brought it new or second hand?
One copy of the game may exchange hands a fair number of times and the developer only gets money the first time that copy is sold. This is one of the reasons why 'Direct to consumer' games are a appealing retail outlet (Live, PSN, Steam, etc) because second hand copies don't exist in that market space.
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