One of the things that the Epic Level Handbook talks about is the fact that when players reach epic levels, the gold piece limit of most cities will be too low for them to purchase many of the magic items appropriate for their level. To rectify this problem, the book suggests that GMs adjust the gold piece limit of cities accordingly.
Personally, I wasn’t entirely on board with this suggestion. I felt like high level magic items should be special; epic, if you want to use that term. I felt too that if I allowed players to purchase high level items in any city or metropolis, it would cheapen the uniqueness of those items. That said, I understood why the book made such a suggestion. From a game balance perspective, if players cannot purchase high level magic items easily, they’re going to have a difficult time combating against epic level monsters. Still, I wanted to make it clear that the ability to purchase these higher level items placed the players on a whole new tier, and I wanted to reflect that somehow in game.
That is the main reason I created the city of Haven. I felt that if the PCs are going to purchase epic level magic items, they should only be able to do so in a place that feels epic. I feel that in creating Haven, I accomplished that. After all, there isn’t much that feels more epic than an intelligent, extraplanar city that can reach out and kill you if you don’t obey the law.
By the way, if you ever decide to run an epic level campaign, and you decide that you want to prevent your players from purchasing high level magic items in regular cities and metropolises, I would strongly advise that you not withhold a city like Haven from your players for several game sessions. It may work well for your story, but in the end, no matter how much time and energy you put into creating unique NPCs for the players to interact with, the players will ignore them completely, and spend the entire session combing through the core rulebooks, searching for the magic items they’ve been drooling over for the past two levels. Until they’re finished with this process, there’s very little that you, as a GM, can do to recapture their attention.