Digital distribution. Just like mobile gaming, you buy the game online, download it, then you can't sell it on to anyone else, because there's no physical media to re-sell.
That said, I don't look forward to having to download a blu-ray's worth of data on the next games like GTA5 and Max Payne 3. But possibly the solution is you open the games up to pre-ordering, and users can download them over a couple of weeks, and then on the release day, the copies are unlocked when connecting to their servers.
Or, commit more to DLC. So you can buy the physical game, but you'll need to keep the disc around if you want to play the full game, as 70% of the single-player gameplay, and the multiplayer, all gets downloaded from online. The disc can hold all the music and the high-def textures, but the gameplay content gets downloaded and only works keyed to your account, or something.
I know the 2nd hand market is a problem for the game developers. But I don't think many of them have done much about it other than whine "it's so unfair!!!"
There are definitely ways around it, they just need to get us players into the habit of downloading more of our content in a means that makes it very complicated to then re-sell on.