"The price" is just a convenient way to talk about the theoretical "value" of a company. Brokers use it to determine whether or not margin maintenance requires

"The price" is just a convenient way to talk about the theoretical "value" of a company. Brokers use it to determine whether or not margin maintenance requires are being met, but in reality there is no single price at any given time. There are always two prices, the highest bid and the lowest ask, and those two prices are only meaningful for the size of the bid/ask in the order book. How would an algorithm determine the price? The whole point of the equity markets is price discovery. The price should rise when people rush to buy, and it should fall when people rush to sell. In many cases that is the "right thing," in the sense that the rush to buy/sell is in response to a real change in a company's circumstances (e.g. an outstanding earnings report, a natural disaster that harms the business, etc.). Limiting the execution of a large order would not stop insider trading, because you do not need to go to equity markets to buy/sell stock -- you can enter into a private agreement with someone to transfer shares in exchange for money. Limiting large order executions would also make index funds impossible -- index funds have to trade the stocks that make up the underlying index when customers buy or sell, and a large fund may have many individual customers buying or selling on any given day. Not all problems can be solved with technology.