Well that is why there is a thing called settlement date or T2. If you have a cash account you know you can't trade with unsettled funds. It takes two days after the actual trade for them to become settled. You can buy something with unsettled funds but you need to wait for settlement to sell otherwise your account will be restricted if you try it. That is why most traders have margin accounts to get around the T2 rule. No waiting for settlement is required.
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