When buying used in Japan, a game is either rated as "like new" or "junk" and is priced accordingly. Of course, all that it takes to put a "like new" game in the "junk" category is a missing instruction manual or damaged box. If there is any actual problem with the game—like a scratched disc, for example—a used game store won't even buy it back. Yet, just to prove the disc is up to snuff, you must look at and approve of any game disc before the store will sell it to you. Thus, when buying used in Japan, you can get even the newest games dirt cheap if you don't mind a lack of instructions or the original case.
Also, due to the fact that renting games is illegal in Japan, most stores buy games back at a higher price than in the US. When I sold back New Super Mario Bros. 2 right before my trip, I only lost about $10 total, putting it about even with a two-week rental in the US. Buy back prices will also tend to stay much higher for much longer in Japan.
Another nice bonus of buying used in Japan comes from the fact that many Japanese fear the internet. Thus, a large amount of Japanese systems are not even connected to it. This means that most DLC bonuses and other codes are never used and you often get the full new game package despite buying used.
So buying used is a very different beast in Japan. If you don't care about boxes and manuals, you get great games for a fraction of their normal price. Moreover, if you treat your games—and your systems for that matter—well, you can easily get a good price for them months or even years after their release. Treat them bad, however, and you're almost better off throwing them away.