I think that you absolutely right in terms of modern economics. However, I don't think that most D&D games ought to simulate modern economics.
I'm going to use comic books as a model, because I am co-owner of Golden City Comics in Toronto. In the modern sense, a handful of old comic books are potentially worth a fair sum of money. However, they are worth absolutely nothing at Radio Shack. If you need something at Radio Shack, you would have to have a canny salesperson indeed to realize that the Amazing Fantasy #15 you are holding (1) is worth far more than the item you wish to purchase, and (2) is not a reprint.
In the world of D&D, though, a sword is as valuable in Hommlet as it is in Verbonc, as it is in Waterdeep, as it is in Sigil. Depending upon the condition of the gear, there are few places that cannot use arms and armour. The local economy may make a big difference in the price you are offered -- trying to sell meh swords in Toledo, Spain, is simply not a good idea.
It is difficult to imagine a D&D world where a sword or a suit of armour doesn't retain strong barter value.
The earliest "money" we know of was made of clay, and contained images of the items that were represented by the "money". "One cow" had, in effect, a stable value, even though two real cows are not always of equal worth.
Likewise, in some worlds, guilds might set the range of prices for which items can be bought and sold within a precinct. Then it becomes law that swords sold within the City of Greyhawk to the Swordmaster's Guild may be bought for X-Y gp, and sold by the Swordmaster's Guild for Y-Z gp.
Back in modern times, if you brought Amazing Fantasy #15 into Golden City Comics, I simply wouldn't have the means to give you 20% of its book value. That is one valuable book. I would be happy to sell it for you, giving you 45% of the take at no risk to myself. But a canny seller would use something along the lines of an auction house or ebay to capitalize on the book's value, possibly making far more than 50% of its cover.
So, within the context of my own campaign worlds, going back as far as Holmes Basic, there are folks who will buy some gear at par, quite a few folks who will buy at 50% (many retailers buy from wholesalers at 50% IRL), a few folks who will pay over par for some reason (if you can figure out who they are, knowing those people is a "treasure" all by itself), and more than a few people who will pay less than 50%.....sometimes 10%.......sometimes paying thieves 10% when you say No......for what you are trying to sell.
I actually find the "What are we going to do with this stuff?" part of the game to be fun, as player or DM. When the lair of an ethereal filcher is located in my world, it is not so much filled with magic items as with the sorts of things that mundanely disappear, a lot of junk, and thousands of mismatched socks........amid which the treasure is waiting to be recognized for what it is.
YMMV.
RC
Actually both he (being Harr, I agree with you RC) and peelseel are wrong, in general. I see people selling things for more than 20% all the time. Plus liquidators, like peelseel, are all there to get it at as low and profitable a price as possible to resell in a retail environment.
Plus you need to look at who your going to be selling to. Weapons, armor, magic items are not going to be sold to the common merchant. They are going to be sold to merchants with ties to the nobility and government. They need weapons and armor. If they can get good serviceable items for 50% rather than 100% they will happily go for 50%.
So I can see selling for 20%, but that depends on how savy the party members are. If they have any understanding of the market they are going to sell as close to the end consumer in the market chain as they can.
So I can see 20%, but I can also see 100%.
Why 100%? Because it is actually a simple matter to hire a full time seller for your recovered goods. Someone you pay a fixed wage to and offer your protection for doing your business. If you have a cleric in the party you can even offer awesome health care benefits to your employee and his family.
Now groups aren't going to do such things all the time or right away. It depends on how market savy they are, and how much value the items they recover have. Sooner or later they will be making enough money to hire a full time seller though. So unless they never figure out how much money they could make off of those hundreds, eventually even thousands, of weapons and armor, they will engage in a "business" where they can sell their stuff at 100%.
Which is good, because it opens up tons of adventure scenarios for the group. Sice it will be "personal", protecting their own business, it will have an extra depth of meaning and importance to your players. Its amazing how many notches your game kicks up when it becomes personal.
Now let me address the foolish idea that a Decanter of Endless Water is a "throw away" magic item. To me its one of the best. Especially if I am in a game where I am able to get it "Blessed". A Decanter of Endless BLESSED HOLY Water is a weapon of mass destruction against undead and even demons and devils. Even so, there are many situations where the undead are down in crypts, where water easily pools. Water that can then be blessed and become Holy Water.
Another cool use. An Orc lair down in caves. Put that Decanter on Geyser and you soon have a bunch of drowned Orcs.
Now say you have a town or city that gets their water from wells. Or worse, from a greedy order of clerics who charge you to create water at a very stiff price. Or worst of all, is in a very dry area where they gather their water via collectors of condensation. Those Decanters are very highly valuable to those communities.
Heck, I even have a city in my campaign world that is out in high desert mountains, but the city is lush and green and makes a fortune selling their fruits and vegetables to surrounding desert communities in trade. All because this cities ruler accumulated several Decanters of Endless Water.
So the Decanter of Endlesss Water is far from a throw away item, unless the players are very narrow minded and short sighted.
Plus there is a simple reason why magical items will never sell for less than 100%, unless it is a charged item like wands. They are relatively rare. There are millions of non magical swords in the world. Probably 100's of thousands of masterwork swords. There are only thousands, maybe 10's of thousands of magical swords that the millions of warriors around the world would like to have.
Unlike modern cars, homes, or even normal and masterwork swords, these magical swords can last for a thousand years. They are harder to break, harder to destroy, or ruin. So demand will always be high among communities who need to reliable defend themselves.
Not only that it is a very reasonable assumption that there are many other up and coming adventuring groups, groups who likely have more gold than they have magical weapons and armor. So they will be more than willing to buy a more successful groups "cast off" magic weapons and armor.
Now a segment of the market where 20% is feasible is wizard only magic items. They are a much more limited market. Where as you have millions of soldiers around the world you may have only 1,000's of wizards around the world.
Now it would be simplest to just assume they only make enough to barely meet demand for those who can't, or don't want to take the time, to make items that only wizards can use.
Which is actually very likely. They are far more likely to make boots, cloaks, rings, armor, weapons, etc... that those 1,000's of warriors would want to buy as soon as they could. So a wizard is far more likely to only make items for themselves to use when it comes to wizard only items. Sure, they get the occasional commission from other wizards to make them such items, but at least 50% of the time the wizard will only have, and take, the time to make items for themselves. Probably financed by all those weapons, armor, boots, cloaks, rings, etc... they made for those millions of warriors to fight over.
So making the assumption that there is, or barely is not, enough such items to meet demand is very plausible.
So even though 20% can be realistic, it depends on the position within the market chain you are selling at.
Remember, the only reason Adventurers can make such huge profit margins on these items is they did not pay to have them made. They did not buy from a wholesaler or retailer. They have ready made products that they have inherited due to risking their lives in bloody battle. No real cash investment whatsoever. So whatever price they sell at is still 100% profit for them.
So if they have accumulated 100 good long swords that retail at 15 gold, the closer to that 15 gold they can sell, the better their profit. Its much better to make 1500 GP than it is to make 300 GP. The more consistently your able to sell closer to 15 GP the better.
Now there are all kinds of details a DM can go into to lessen these profit margins. You can give each piece a "condition", which infers reliability, or unreliable. Then alter the price based on these conditions. Who the heck wants to track the condition of what will eventually be 1,000's of items in a campaign?
Not me!
So going on a "average" assumption of 50% is much easier, and much fairer, in the long run.
Magic items should always get premium prices. These are rare items that WILL make the difference between life and death. Success or failure. So they will always command top prices.
Obviously exceptions can always be negotiated or created. However, unless you want to keep such details recorded, a general and averaged rule of thumb is much easier, and much fairer, to those playing the game.
Otherwise your going to have a player come along and realize what baloney 20% is and make you record "conditions", etc... so you can keep them from getting 100% of retail as much as possible.
I guarantee you if I ever play in a campaign where only 20% is given, that DM better be prepared to keep me from making a fortune. Plus they better like such details in their games, because their game is gonna have it if I am in it and they only give 20%.