But if the player can loot foes, if the foes drop worthwhile items, if shopkeepers pay fair prices, and the shopkeepers have enough money, then the player will end up a tycoon about a half hour into the game. The problem all traces back to the conceit we all take for granted: Players can take on wave after wave of willing enemies. Never in history has there been a gunfighter or a swordfighter that regularly and single-handedly killed batches of foes with impunity. If there was, people would have stopped fighting them.
When the gameworld generates foes, it's also generating gear for those foes, thus poofing into existence a huge haul of valuables for the player to obtain. No economy can remain stable if a single person can supply an endless stream of valuable goods from thin air. The player becomes a loot-generating machine. They are like the replicator machines in Star Trek, except they can't stop producing stuff. (Unless they stop playing the game.)
There is no way to patch this economic perversion to have it make sense. Trying to make a simulated economy on top of this absurdity would be a waste of valuable programmer time, although it would lead to some humorous situations: For example: A longsword is a valuable tool. But as you sell more longswords and they get spread around the gameworld through trade, the prices for swords would, in a detailed simulation, fall drastically. Once everyone in the world and their grandmother has a sword for each hand and one for over the mantle, the world will no longer want any swords. Blacksmiths would stop making them. There's no way they can compete with you, since you're essentially a cost-free sword factory. The price of swords would then drop to the price of the raw materials they contain. People would buy your swords (and you would still sell them, since they cost you nothing to acquire) just to melt them down to make other stuff. At which point you would be putting miners out of business, since it's way more expensive to prospect, dig, and smelt, than to just buy a wheelbarrow full of metal swords from the player character. In the end you end up with one single person driving the blacksmiths, cobblers, miners, and clothiers out of business.
The best solution for dealing with the oddball behavior of an in-game economy is to just ignore the silliness and enjoy the game. The worst thing you can do is waste time over-thinking it.
Like I just did.