Many electronic high-frequency trading firms follow more or less market-making strategies, where they try to buy stocks for $9.99 when people want to sell, and sell them for $10.01 when people want to buy, and balance their buys and sells to make lots of tiny profits without much risk. If you do this in the public markets, you run the risk of what is technically called “adverse selection,” or less technically “getting run over”: You might trade with someone who knows something you don’t, or who is just in the middle of buying or selling a ton of stock, and the price might rapidly move against you.