The market has AI machine learning algos that front run your order and sell (or buy) to you at a higher (or lower) price. The use speed and pattern recognition to get in front of what you want to do & collect a spread. If you try a trick these algos/machines you will be charged With a crime called #Spoofing. The exchanges are pro algos and help pass the anti spoofing rules/law. Why? Because robots create more brokerage fees. Here's an example you go to buy a share and place a bid, the algos recognize your new "real order" and lift the offer