In-Game Economy

An economy is a complex thing, becuase there are so many interacting agents. There is a reason economists have lots of complicated mathematical models. For a fantasy role playing game, where the goal is to have exciting stories and adventures, all you normally need is some prices for goods characters want to buy or sell, typically adventuring equipment, later on maybe costs for armies, and building keeps.

For selling it is useful to have a discount rate by item type: how much cheaper than what they sell it for would a merchant be willing to buy something for? You need this for the types of things the adventurers win in adventuring. I typically use

• Coins: 95% Obviously they can pay with 100% of the coin value, but if they want to exchange coin types, for example a heavy bag of 1,000 copper pieces for a platinum piece, there will be a money changer's fee of 5%. 
• Jewels: 95% The same applies for exchanging coins into jewels for lighter transport. I allow to pay with jewels at full value, just as with coins, but you could charge a 5% discount even for that.
• Trade goods: 80%It does not make sense to buy goods at the same price you'd sell them, so instead of 100%, I give 80% for goods on the list of trade goods. This allows at least to reasonably have some level of trade, even if the value is still high. Bars of copper, silver or gold would be treated as coins or jewels at 100% to pay with, and 95% to exchange. 
• New weapons, armor and equipment: 50%  you need to find a merchant trading in them, and they will pay half price, if they are interested to buy. If the characters have too many items (say, hundreds of swords) that they will have a hard time selling again near term, they may only buy a limited amount or ask for a lower bulk price. Note that it is very rare to find new items -- maybe sometimes in a storage room or armory. Most items are being used. 
• Used weapons, armor and equipment: 0-25%Especially for monster derived weapons and armor, that would be ugly and not fit in style and balance to human needs, it often will be close to 0%. For equipment, or human weaopns it might be between 25%, most commonly I use 10%. 
• Art and Jewlery: 50% This is a bit tricky, as Jewelry could be broken down into jewels and precious metals, that could be used to pay at 100% or 95%. However, the value of jewelry is often a multiple of that of the pure gem and metal value, due to the art and workmanship that went into them, just as a beatuiful painting is many times the value of the paints and canvas. I simple use the same 50% as for other goods. 

The economy in the game as described does not work, really -- for example trade goods always cost the same to buy and sell. If that were so, there would be no traders or merchants for them, because without a price difference and profit margin, they could not survive. Why would you pay for horses, carts, drivers to transport grain or metal wood somewhere, when you cannot sell it for more than what you bought it for? And why would you maintain a stall or shop doing so?

In a real economy, anything that is produced must be sold more dearly than what the input materials cost, an what the living costs of any labor to produce it cost, or it could not be produced in the long run -- the manufacturer would go bankrupt.