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If this is a core portion of the game design, I've seen it labeled as Player-Generated Economy.
For example Neopets where players can make good money by buying limited items on the cheap from the official shops and reselling the items at a profit to other players.
And also Puzzle Pirates where the easiest way to get raw materials (wood, iron, etc.) is by posting buy orders for AI-controlled merchants. Usually the materials are worked into finished goods, but you can try to find someone willing to pay more for the wood and iron than you bought it for.
Edited by KD
Bob buys an item for 15 gold in shop, Sell price to the same shop: 5 gold Sell price (value of item) to another player: 1067
Edited by Baolen2445