When the market is at 70-80%, buy out a store's entire stock of expensive items. When it hits 120%+, sell everything back.
: Certain glitches involving stacks allow players to duplicate items in their inventory, which can then be sold for infinite profit. my time at portia save data unlimited money upd
For a more automated approach, several third-party tools provide "Unlimited Gols" toggles. When the market is at 70-80%, buy out