Get ready for the following assumptions from people who want to tell you how to play:
1. D&D is not about economics. You suck for even thinking about having economics in a D&D game. Go play Monopoly.
2. Economics is a complicated voodoo that adventurers can never hope to understand. Talking to another person, and making a deal is super complex. In a feudalistic setting where chain stores do not exist, and inventory is mostly owned and sold by the merchant, the merchant will not barter. Bartering is non-existent in such settings.
3. Magical items can’t be bought, sold, or traded . . . they are special in that way. Everyone wants them, and no one would ever want to sell one, or buy one. Unless you are an adventurer.
4. Only adventurers use magical items. Guilds, militaries, lords, nobles, gentry, and merchants don’t want magical items.
5. Only adventurers have gold to afford magical items. Guilds, militaries, lords, nobles, gentry, and merchants don’t have enough gold to buy magical items.
6. One cannot approach guilds, militaries, lords, nobles, gentry, and merchants and offer to sell magical items to them because they don’t want them and they can’t afford them. One cannot even find other adventures to sell or trade stuff to.
7. Adventurers don’t care about money. But yet they have more money than guilds, militaries, lords, nobles, gentry, and merchants.
8. Adventurers don’t have downtime, they adventure all year long, even in the winter.
9. In fact adventurers don’t have wives or husbands or families that they spend any time with, unless they are also adventurers.
10. Everything from hotdog buns to holy avengers sells for 20% of there trade value. That is if the GM lets you sell the item for 20%. Some items are special and sell for 100% like gold daggers . . . but magical items are not special.
11. There is no such thing as supply and demand in D&D.
12. Economic rules would be unrealistic, so don’t bother. But we should bother with unrealistic combat rules.