1) Some people know how to sell stuff. My cousin can sell anything, it’s amazing. People want to buy crap, some people know this, some people understand this, and some people can read peoples reactions. Saying that an adventurer with a high intelligence, wisdom, and / or charisma can not figure this out is posteriors.
2) There is the real value of something, and than the value someone tries to sell it for. They are not the same things. Initial public offerings of stock are like this. Some items you buy at the store are like this, like most cars. Some items have a demand that drives the price people are willing to pay for something close to the price companies sell things for. Take the Toyota Carola for example. You can buy a new one for $14,000 or buy a used one for $13,000. That just rocks. The company prices it close to the demand, and the demand is close to the price the company sells them for.
2b) Put mostly anything up on eBay, and you will get the real value for the item. This is almost always less than the price you can buy the same item for in the store. There are exceptions, and there are folks out there too stupid, lazy, or carefree to use Google and find the same item cheaper on another webpage, or brick and mortar store.
3) Supply and demand. It goes two ways. If something is worth a whole lot of gold, then people with a whole lot of gold want it. A demand for something means selling that something for what that demand is . . . is possible.
4) Spotting people who make up the demand side of the equation in a feudalistic economy is not voodoo. They are the guilds, churches, merchants, gentry, nobles, and royalty.
4b) Adventurers are they rise in power should meet these powerful folks who belong to the demand part of the magical economy (guilds, churches, merchants, gentry, nobles, and royalty). One would think they would talk about common interests like magical items (if it was not all ready obvious to the adventurers).