I'd say let's decouple the buying price and the selling price. The buying price should have a lower limit and an upper limit, the exact price should be determined by the demand (the higher the demand, the higher the price). The selling price should have similar limits, but it should be determined by scarcity/supply (the more an item is sold, the less money you'd get). Also monetary transactions should be bound to contracts (an exchange of similarly valuable goods or services), it shouldn't be possible for rich individuals to give everyone a million bucks without anything in return and crash the economy.
