I think that's overstating the impact of these people. (Day...

I think that's overstating the impact of these people. (Day traders, particularly those who frequent hotcopper). Perhaps there's some of that going on but the average person doesn't have access to sufficient capital to either make this worth the effort or to impact the market directly. This is primarily because most brokers (for retailers) require settled funds for orders, whereas brokers, institutions need to pay up on settlement. Combined with the fact the fees/brokerage problem isn't an issue, there's no disadvantage of playing both sides of the market. (Whatever their objective - to get out with the best price, or to get more shares as cheap as possible). I know for a fact this happens since I worked at a broker.