I don't think there is any technical problem with this, but often you will need to have the cash to cover the buy, because you won't have access to the sell funds for a certain period of time. So you can't flip the sell proceeds directly into the next buy, but there's nothing to stop you from using other funds to buy the same stock you just sold. People who day-trade have "margin" accounts for this reason. Their broker essentially loans them the money to buy the stocks. Very risky if you aren't 100% sure of what you are doing.