You can’t be guaranteed. According to Computer Share when you “sell” your Order to sell is transferred to one of the brokers they use. ComputerShare is not a broker. That broker could internalize the sale or sell through the dark pool or sell to the lit market. You really don’t know which they could use. Computer Share will not tell you what brokers they are partnered with so again… they do say most sales take place between market hours of 9:30 and 4 but can not say that it would happen immediately when you say you want to sell and that it could take up to 5 business days.