The ecology of something shows the thing and how it fits into the systems of the world. So we have magik items in the world, lots of items. I can accept that, if magik has no mystery and is just another craft like baking, smithing, or farming. However, I have to ask... Who is making them and why?
How many NPC magik users are in their little workshops cranking out items? First how many MUs are there in the population (rather than the adventuring population)? 1-100, 1-1000, 1-100,000? Given your average large medival tech city has a few (1-3) thousand people, supported by a near equal number of people out in the fields, (assuming an excessive 1 in 1000 people have magikal talents outside the adventuring population) means you will have 6 magik users in a good sized city. How many of those people are working for a specific patron (who may or may not have them busy making items... or are they doing magikal work for the patron)? How many of those people are working for the city as laborers (magikal lamp lighters... how many of those spells can they cast a day?) or as magikal guardsmen (We will take the orcs, you take the shaman)? How many does that leave toiling away churning out items and potions and other mystic nicknacks that D&D GMs like giving away or allow you to buy? Lets figure 2-3. How many large cities should a country have? Normally it would be 2-4. So we will consider a country will have 8 people in cities cranking out items. Those living in other places probably have their own concerns, but they might contribute a few things.
How long does it take to make some of these things? Potions used to take 4-weeks to make (healing), Swords 1-3 years depending on the plus, wands 1-2 years, Wizard's Staff 10 years. Someone can correct my times, but lets work with those.
So you are making potions in a hope that some fool adventurer will buy them. You can make 12 a year. Cost of living that you want to have (upper middle class) divided by 12, plus the cost of ingredients (shaving a little of market price because you are buying in bulk), plus 10-20% profit is how much a potion should cost. Insert the various costs for your campaign world. Say owm at the cost.
Now that is if you sell 100% of your wares over the year. If there are not that many fool adventurers, figure you will sell 2/3 normally, so you will be taking your total costs (lifestyle + costs of ingredients +profit) and instead of dividing by 12, divide by 8. It is a good thing these things are found in dungeons because they get pretty darn expensive to buy.
Note: This was not supposed to be a lesson in economics, but it shows the effort that people will have to go to make items. If there is no money in an item, nobody will make it. For example, a mage spends two years making a ring of spider climbing. So that is the cost of 2 years of their life, plus the cost of the ring plus all those spiders. If nobody buys that ring, the magik user is out all that money. You might make the item as a personal challange, but that means you had to make all that money before hand. If you are not adventuring, that is a lot of potions, notions, and lotions.
If all your free magik users, in all your cities, in your country all made potions that took 4 weeks to make for a year you would have approximately 96 potions. Note: That is if making healing potions was all they did. If they were doing other things, or took vacations, the number should be around half that for potions or 48.
How many potions did your adventurers swill down in the last year of play? The last D20 group I interacted with, quaffed about 66 over their last year. Your numbers will vary, but you get the idea. A group of adventurers or an elite unit of an army will polish off most of a countries stock of potions in a given year. So most of these free magik uses are probably making potions/ lotions/ and notions, rather than items to keep up with demand. So figure 3/4 of them are making consumable items.
How many party of adventurers are there out there? Is it just you guys? In all the world? In your country? (and if it is, why is anyone making things for adventurers?) Given the number of high level people (requires adventuring to do that last time I checked) there must be a lot more than just you guys. Especially if there is a trade in armor and weapons for non military types.
Another aside: Who is dedicating weeks/ months/ years into the creation of items that they can't use? The Magik sword is a great example of this. (There are non magikal plus swords, as listed above). All it does is make the fighter a more important part of the party, instead of the magik users who should be. Magikal items usable by mages/ wizards make sense, they are "tools of the trade" so to speak. Commissioned items makes less sense, unless the character makes enough to buy the components, make the item and make a profit (figure 2x the cost of the time to live and component costs since it is commission item after all). Who has that kind of cash? Governments, Royal Nobles, and a few Merchants. The theives guild would be doing quite well if it could support an MU to make one magik item. (of course blackmail and other things could be applicable). That is unless MUs don't like to live in any kind of lifestyle. If you have magikal powers would you live in total sqallor? I didn't think so.
As discussed above, Nobles, Governments, and some Merchant houses will have the cash to get magikal toys for their employees. So how do these things get lost and put in dungeons/ places to adventure? If I spent that much money on something, I would keep track of it. Imagine the US army not knowing where all its multimillion dollar tanks are? They know generally where they all are. Let credit the governments of your worlds that much creditm "Okay, so He died in the swamp. It must be in there. " (magikal weapons and equipment existing in treasure taken off dead adventurers seem okay). Do you not think they will put their mark or measure on the items being made (the local equivalent of "Property of US Army stencil)? If you were found walking around with such an item, do you not think they would not ask for it back?
Of course some of these items get eaten, disenchanted by anti-magik monsters, thrown into pits of doom, and so on. Figure a small percentage (1%) are destroyed or made unuable by circumstance.
So lets put it together. Taking into consideration that not everyone is as mad as Corvus, you will have a handful of low level, non adventuring magik user types cranking out potions and mystic nicknacks that people will want to buy (or that they as magik users might need). Most of the ingestable items will be eliminated in the year or so of their creation. Permanent items will last nearly forever, figure that every year a dedicated group of magik users will make half a generic +1 sword or 1/6 of generic +3 sword (given my times), if there is enough money floating around to warrent its cost. So over 100 years a country will produce 50 or so generic +1 swords. Of course if they generate all those swords, there won't be all those other cool mystic nicknacks.
The point of this little tyrade is to illustrate that these things don't "grow on threes" that there needs to be a reason that these things are being made. That only so many can be made at a time. That over time, the number of items can add up, but that people will still want to own their magikal item.