Flow of Goods Rule:The quality of goods in the world is dependent upon the shop's distance from the final dungeon. It doesn't matter if the town you start in has a huge thriving economy and is the center of world trade, it will always have the game's worst equipment; and even if that village near the end is isolated and has only three people in it, it will have the game's best equipment.
The Best-Laid Schemes:The final villain's grand scheme will have involved the deaths of thousands or even millions of innocent people, the clever manipulation of governments, armies, and entire populations, and will have taken anywhere from five to five thousand years to come to fruition. The hero will come up with a method of undoing this plan forever in less than five minutes. (Compare Spanner in the Works)
Pyrrhic Victory:By the time you've gotten it in gear, dealt with your miscellaneous personal crises and are finally ready to go Save the World once and for all, nine-tenths of it will already have been destroyed. Still, you've got to give your all to save the remaining one-tenth.
Adam Smith's Corollary:No matter how thoroughly devastated the continent/planet/universe is, there's always some shopkeeper who survived the end of the world and sits outside the gates of the villain's castle, selling the most powerful equipment in the game, like nothing ever happened.