When you sell shares, it takes 2 business days for the money from the sale to settle.

Before the money settles, you can actually use the unsettled cash right away to buy another stock if you wish to.

Be careful of good faith violations when you do this.

However, if you wish to withdraw the money instead of reinvesting it, you will have to wait for 2 business days so the money can settle and become available for withdrawal in line with U.S. SEC regulations.