In the pre-HFT era there were manual day traders doing spreading, arbitrage and short-term speculation based on the order book. A famous example would be the SO

In the pre-HFT era there were manual day traders doing spreading, arbitrage and short-term speculation based on the order book. A famous example would be the SOES bandits of the 90s. They would pick-off slow NASDAQ market makers and offer the shares back out on nascent ECNs like Island to make a few ticks. Some of them worked for firms as professionals, but many traded on retail platforms similar to what you'd see from Interactive Brokers or TT these days. Still you are right that the average investor never traded in this fashion. At worst, the people HFT put out of work were trading as a profitable hobby similar to playing poker online.