A trademark's protection starts from the date of filing, not the registration date. So, if products are already in stock, there is no reason to wait before selling. Your unregistered application already provides a certain level of protection.
However, that protection is only valid if the application is ultimately accepted. If the application is refused, you will need to rebrand. Acquiring large quantities of stock before the outcome is confirmed carries that risk; if a refusal occurs, significant stock with the original branding becomes a complication.
To avoid the risk of rebranding, it's generally recommended to submit your name for a lawyer's check that will uncover any identical or confusingly similar names.