There is a pretty interesting post regarding the distribution of magic item types in the game world and some of the complications therein on the official forums. Here's a taste:
1. If you take a look at the Dungeon Master's Guide it explicitly has built into the weapon tables heavy bias towards some weapons over others. This is because D&D is at least partially built as a medieval reality simulator in addition to being a pure game, and well the questing knight would be finding a lot more longswords than he would repeating heavy crossbows.
2. Some weapons are just more popular than others. There will be a LOT more people specing in Longsword than in Kamas, and these people will be just as upset if they're drowning in Kamas but can't find enough longswords for all of them.
3. People who spec in rare weapons though, also want to be able to get their rare weapons sometimes, even if the 30 other fighters that went through a dungeon before them all were looking for longswords.
Within the same thread, there is also a post confirming we'll have pretty much unlimited trading potential, the the exception of some bound items.
