The reason for the trade restrictions is to prevent players being helped to advance at an artificially high pace. It also obstructs the related problem of players selling goods for out of game currency (such as money). Of course, allowing higher prices for higher age goods shouldn't be a problem in this regard, nor do I believe that it would be a major technical challenge. It mainly requires some pretty basic database operations, and a bit of fiddling with the UI. Possibly, they're thinking of the need to trade in stages as a game feature. It certainly does mean you have to plan ahead, and it is a strategy game, after all.
In my opinion, the ideal would be to open the trading up entirely, allowing it to become a major game strategy. However, that would require another way of preventing abuse. Unfortunately, I can't think of such a way, and neither, it seems, can Inno.
The notion of holding NPC traders to the same rules as players is meaningless, though. It would make much more sense to remove them entirely. The idea behind the trading is to create a need to have friends and guilds, and to interact with neighbors. We're simply not supposed to use the NPC traders; they're for emergencies only.
Whoever had it is either further on or gone from the game.
This doesn't make any sense, Shy Ted. The NPC traders are a game feature, provided by Inno, and Inno neither moves on nor leaves the game. Barring a major bug, the traders should be there. They'll be at the end of the trades list, though, and won't show at all if you're using the neighborhood or guild filters.