I don't think any of my players have even considered the possibility of buying magic items.
IMC, arcane spell users are held responsible by the bulk of the populace for the dark times two centuries ago that saw gates to the CE planes open deep in the underdark. Since the gods also withdrew from influencing the world at this time (although they are back now), they have fallen out of favour with much of the populace, and divine casters are general looked upon as misguided nuts.
A weapon or armour dealer may in a really big city may have some minor magical weapons or armour (+1, possibly even +2, although virtually no chance of an obvious magical effect).
Since most people with power or who are well educated realise the common views regarding magic and the gods are silly, there is a magic-market, but it is not common knowledge, and tends to rely on having contacts in the right places.
The PCs just become involved with a group of sages who, unbeknownst to them, happen to be loremasters and clerics of Boccob. At a later stage, it may become possible for them to commision magic items on the odd occasion from this group, or another one.
I think that if something approximating the default treasure rules is used, the availability of magic items for cash, to some degree, is one of the few ways to actually provide high level PCs with a constructive use for their hard-earned wealth.
