The thing is you can beat a game (assuming multiplayer or high scores or whatever isn't the main objective). Theoretically an arbitrarily large number of people could receive the full benefit of the game from just one purchase. In contrast, (most) people don't beat the need for transportation; if you sell your car you'll need to find a replacement. Who knows, you might even go to the same dealer. And since people tend to keep cars much longer than games, a car might change hands one or two times before reaching the end of its usefulness, while that Final Fantasy NES cartridge could still be floating around within your group of friends 20 years after you bought it.
Of course, games do go out of print and companies do go out of business, so I'm in favor of allowing people to buy used games if solely for that purpose (although digital distribution services like Steam and GameTap are starting to solve that as well). But otherwise I try to buy new when possible in order to support the developers since, from a financial standpoint, buying used is really no different to them from just pirating it. Even renting would help them more.