I think second hand sales do more damage to the games industry than piracy. At least with piracy, you aren't competing for the same space in stores that the copies that will make you money occupy.
Stores like Gamestop make the vast majority of their profit from reselling games that the developer receives not a single penny for. They do this because they can buy second hand games for pennies and re-sell them in their store alongside the brand new stock but make them $1 or two cheaper.
Pirates have already made the decision to download the game for free, so they were never likely to give you any money, but the people in the game store were there to spend money but end up giving all their money to the retailer and none of it goes to the developer.