While there is a lot of power behind a system like this it is important to understand some basic fundamentals about how the items, and the quantities you set affect the long term value of your game's internal economy.
It is a balancing act, and is not something that can easily be achieved, or explained in a few paragraphs. Try to find a balance in the amount of items created vs the amount of items removed from the game. Items will be added to your game in a number of ways, from loot on NPCs, to loot in containers, or crafted, or rewarded from quests. Everyone loves to get a bunch of loot, but there needs to be ways to use the loot. The hype from receiving loot will wear off fast if your player has no way to use them, especially if their inventory fills up.
For every way you add items, there should be a nearly equal way to remove that amount of items from the game. In this example world I am providing you with a Trader NPC to sell items to, the trade off is it produces currency.
The trader tries to balance this offset by also selling items to the player, and the amount the trader sells items for is always higher than the value it pays for them at. This is why it is important that you sell stuff the player actually needs and can use. Consumables are the best, because after use they are removed from the game.
I am also providing you with a crafting station, which if used right you can use it to pull more items out of your game then it is producing, but really what it is doing is combining the value of multiple items into the value of one item.
Your goal should be having the player's wealth (as a measure of all items and currency) steadily grow as they progress, it shouldn’t grow too fast, and it shouldn’t fail to grow at all. Failing to grow wealth will kill a game faster then one where it grows too fast.
The point of this is to maintain the value in your items. When your player gets a rare item used for crafting something special 100 hours into the game they should have that same feeling as they do when they got the same item just a few hours in. Sure it is less of a mystery, but by this point they should have a good idea of what they can actually do with that item to progress in some other way in your game.
This is just the tip of the iceberg, like I said these are some deeper concepts, and a few paragraphs is not going to be enough to guide you to fully understand, especially for a multiplayer game.