It seems to me that repricing of items isn't going to change the fact that PCs will still own and prioritize "the big 6." The only effect is that they may be willing to spend a little more on some of the extras. Instead of getting the Corvette (+5 sword), they may be willing to stick with the Camaro (+4 sword) and get a dinghy, too.
That's
exactly it - you can tweak and adjust prices until doomsday, and it won't change the fact that PCs will buy the things
they find most useful, no matter the price. Increasing the prices of those useful items will only mean that the players will gripe about it, and their PCs won't be able to afford them until higher levels (which, by the time they CAN afford them, might well be obsolete). Decreasing the prices of less-useful items only makes them less valuable, and more likely to be pawned off on henchmen, sold, or bartered away for other, more useful items.
And really, I think Andy got it wrong: the more common, more-often-bought items are going to be cheaper, on the whole, than less-often-used items, because sellers have a better chance of unloading them. This is, of course, assuming that
a) you allow magic items to be bought and sold - I'm not supporting the "Magic Item Walmart" stance (I think that idea's totally absurd), but I do think that, like nuclear warheads, F16s, and M1A1 tanks, there IS a market for such things.
Anything can be bought and sold, if there's even one of it in existence and someone's willing to buy/sell it.
b) sellers aren't gouging potential buyers by pricing things at 5 or 10 times their actual value because they know people will still buy them, because the items are SO useful that the buyers can't go without them (the California Gold Rush is a good example of this).
c) the most useful items are reasonably common. If you're playing in a low-magic world, ALL magic items, no matter how useful they are, are going to be expensive, simply because they're rare. In a normal D&D world, you can pretty well assume that commonly-used items like weapons and armor will have fair to high availability.
"Usefulness" is, of course, subjective - in a world that's 90% ocean and all the kingdoms are on archipelagos, items like helms of underwater breathing, folding boats, and rings of free action are going to be in high demand and would most likely appear a lot more often than they would on a desert world (Dark Sun, for instance). Likewise, PCs in an undead-heavy campaign are going to buy/make items like wands of searing light, holy swords, and ghost touch weapons/armor, because those are more useful to them. But yeah, even in those two cases, the "Big Six" will still be cropping up because they're useful no matter the situation - they possess universal utility, so to speak.
I may have magic shops have an "on hand" limit about 1/10th (Maybe I'll tweak that. S'just off the top of my head.) the gp limit of the city that they're located in, too. It seems unreasonable to expect nothing to be on-hand at Gargool the Great's Imporium of Wonders, after-all...
I like this idea - it makes total sense. Gargool's going to have a good stock of cheap magic items, but he's not going to burn gold/XP (or have his artificer do it) on high-price items just to have them sit on a shelf and collect dust - he's going to wait until someone commissions such items, and THEN makes them. It's just like a real world bookstore - they can't carry every single book that comes out, but chances are they CAN special order it for you.
No, the problem with many items comes down to the slot it uses. Why wear an amulet of proof against poison when the amulet of natural armour is just far better in most circumstances? At this point, the gold cost of the amulet is irrelevant. To fix this, you need another solution.
Carry two amulets and swap them out as needed? I play in an NWN persistent world; I have an amulet of natural armor +2, a lesser amulet of health, and two necklaces that protect against acid and cold. You know which one I wear the most? The amulet of health, because a lot of the monsters I fight use poison and/or disease attacks. Never mind that my Fort save will likely protect me against most of them; I just prefer not to have to deal with it. When I go into a fight, I chug a potion of barkskin - that obviates the need for that amulet of NA +2, which sits in a chest in my inn room. If I'm dealing with oozes or ice elementals, I swap out protection necklaces as needed. Granted, it might not be as easy to do this in a PnP game, but combining effects like you're suggesting increases the price, sometimes by quite a lot, when it would be cheaper in the end to simply buy/make another item that does the same thing and swap them out as needed.
Additionally, it fosters overdependence on one item; imagine if the PC had this great necklace that granted +2 natural armor, +2 saves, and +2 deflection. The PC uses his two open ring slots for less-useful items, but if he loses that necklace, he's going to he hurting far worse than his friend who lost his amulet of NA +3, because he just lost the equivalent of three items!