Actually, it might even be worse than piracy. One pirated download does not equate to one sale as very often the downloader wouldn't buy the game in any case. For a second hand sale, the buyer is much more likely to have bought the game new- after all they've just paid for a copy, but they just wanted it cheaper.
Regardless, other industries seem to manage just fine when their products are resold, and I don't think that games companies are justified in trying to restrict the second hand market by introducing DRM, which they are currently trialling on the PC.