>1) The price differences are due to supply and demand. If there is more demand for a stock, the price will go up. Conversely, if there is less demand, the price will go down. 2) If someone closes their position in Amazon stock, it will be closed at whatever the current market value is at that time. So if the stock was at $100 when they bought it, but then falls to $99 before they close their position, they would lose money. 3) You can make money by buying and selling stocks as long as you buy low and sell high. So even if a stock falls in value after you purchase it, you could still profit from selling it later on if the overall trend is upward (and vice versa). It all depends on timing. 4) All platforms have different prices for stocks because each one has its own order book with different bids and asks from buyers and sellers respectively