To a degree availability likely depends on location yea. That mundane short sword might not be a freshly made by the local smith so much as the local smith who specializes in things like horseshoes nails & cooking pots sending his apprentice to billybob's place cause he found a buyer for that sword he keeps over the mantle from when he got drafted to fight those zombies. Meanwhile somewhere else where the zombies have been attacking every time a particular moon is brightest might have so many swords that a well cooked meal could get you two.That begs the question...
Who is going to buy the sword?
The farmer? He's a farmer. The sword doesn't help him farm. That longsword is of value to the player but not to the village. There's already an armory where there are swords if trouble strikes. The average person doesn't have one and the local lord probably doesn't want the populace to have swords either.
As it stands, I make swords and armor something that is not available for sale outside of cities. There's neither demand nor permission for it. If a PC wants a better weapon or armor, s/he can take it off a corpse or find it in a dungeon. The blacksmith is a blacksmith, not a weaponsmith and certainly not an armorer.
As to the weight-to-value, that's why I let players use hirelings. It gives something to spend gold on (services) and a reason to be in the world.
Skyrim is an okay example. However, even it needs patching to make it work (ie with mods like (surprise!) "Trade & Barter"). And remember, even in Skyrim, the alchemist won't buy a sword from you and the armorer won't buy apples from you. You need to go to the proper merchant.
Ultimately, I'm trying to think of a way to have a more robust (or at least functional) economy in the game and am trying to figure out if it can be patched with barter.
I think that the economy destroying party of PCs walking around the local lord's territory with the yearly GDP of the empire he's subject to if there were two different types of currency. One currency for normal stuff & some sort of magical currency used to enchant things like wizard towers & the kinds of magical stuff players care about. The real world is kind of similar but less overt in that regard.... Sure there are countries that will allow people to buy tanks bombs & military ships from them, but after a certain point a significant part of the pricetag comes in the form of political things. The large number of spells & certain class abilities having Xgp components rather than something more abstract like 2 ounces of fernian dust or something makes accomplishing this sort of thing a huge mess at the table though