I prefer magic items to be scarce and coveted in my campaigns. The magic items' value is to be strictly tied to the potential buyer. We all know from basic Economics, an item's value is derived from its supply and demand for it.
Are the magic items everywhere, mass produced in wizards' labs in big cities? No. Only a handful of motivated and curious enchanters make a life of creating magic items, working for years to create better and more beautiful items, maybe destroying most of their mediocre work through their career. They won't advertise their stock, it draws unwanted attention. They are mainly creating such items for understanding magic and the weave, the art, with the hunger for knowledge. Seldom they will sell their stuff to anybody. When they do, it will be mainly to get raw materials. They will hide their creations. When they die, most of their work will be forgotten, only to be found centuries later by adventurers.
Too few adventurer wizards create magic items, and when they do, it is mainly to keep the items to themselves. Adventurers generally uncover magic items from forgotten crypts, dungeons, ruins etc. But too many adventurers die to monsters' fearsome claws, or to traps in deadly dungeons. So what they acquire will be lost where they fall, or maybe moved elsewhere by monsters, or stolen by evil beings or rivals. Also, too few people decide to be adventurers in the first place. Maybe not even one in a hundred.
In any case, due to such conditions, magic will be a rarity even in a world of wonders. There is no constant influx of magic items to the medieval market. This makes them very valuable indeed. But what about demand?
A peasant or a commoner can't probably afford a magic item ever. Because there will always be better buyers out there for such items. Peasant lives to earn money, livestock to feed himself, his family and maybe relatives. He doesn't care for a magic weapon anyway. How can he use it? He is not a fighter, not proficient with any weapon. A potion of healing? Yeah, maybe that one. He may one day use it when he is badly hurt falling from his roof while thatching. How many times a peasant needs to heal himself? Once a year? He may just buy one if he can save his one month or earnings, and can find a seller near him.
A member of a clergy, or some merchant or a craftsman as potential buyers? Do they want to buy a magic item? Local priests may buy it if the item is a potion, a scroll or a similar consumable that can help people of his faith. They don't have much money, they maintain their place with donations, so can't pay big prices for magic items. A merchant may buy such items to profit. So among all else, they are the ones that will try to bargain heavily on an item. Soldiers? Yeah, they may be potential buyers, they surely need weapons and healing magic. But they are also limited in their actions because they belong to an organized group and under orders. What happens when a soldier finds the money and gets himself a longsword +1? Do his officers permit him to keep that while other soldiers use normal weapons?
The rich and the nobility? Yeah they can afford nearly everything. But will they hoard every simple magic item in their mansions? Set aside the collectors, I don't think so. They will want to chase special items, objects of art, be it magical or nonmagical. The rich people don't blindly buy everything they won't ever need. If the noble meets you and you have a +1 longsword and a beautiful jeweled hair brush set to sell, he may well choose to buy the latter instead of the former.
So until this point your hard earned magic items will be on low demand among the bulk of the population. Who else left as potential buyers? Adventurers, spellcasters, mercenaries, agents of poer groups, villains and the ordinary people that strive to gain more power and influence in their lives. In other words, heroes and to-be heroes and villains, just like you and your party. They desire to gain power, and they want to pay big prices for powerful items, or make someone pay for them (if they are evil or selfish). They think they know how to handle such power, they have the means and the ability to protect their magic items against theft etc. unlike the normal folk.
So, as a result, you eventually end up with a small number of people in the campaign world that will want to buy magic items. You have to meet them and strike a bargain. It is now an adventure and role playing opportunity in itself. Do the books need to give gold equivalent values for the items now? Maybe, but not necessarily. Those prices fluctuate violently from city to city, from person to person anyway. The books need only give neat guidelines to DMs about a logical pricing strategy.
