The introduction of CDBL does not make any difference to the process of buying and selling although it does make a difference to the settlement of such trades.
Where investors have a CDBL account through their broker then the act of giving a sell order to the broker also authorises him to move securities from the account to settle the sale. The broker will move the securities when he enters the order into the market. If the order is executed then the securities are used to settle the sale. If the order is not executed then the broker will move the securities back to the investor's account.
Where investors have a CDBL account through a custodian (who is not a broker) then they must advise their custodian that they have sold as they do for physical securities. However, the securities must be in a CDBL account before they are sold and the broker may wish to check this fact with the custodian before executing the order.
On the settlement date of a bought trade the broker will move securities to the account of the buying investor (provided the investor has paid). Investors may leave the securities in their account (ready for when they wish to sell or to avoid the need to hold certificates) or they may request the participant to rematerialise the securities.