Sellacious dashboard is the only way to add a product in sellacious. if you are referring frontend as https://www.xyz.com
it is still under consideration whether to support it or not.
As creating product require lot of information to be filled thus seller will login into https://www.xyz.com/dashboard
(where dashboard is sellacious dashboard) seller will see the only information he is meant to see, you can control it from permissions (https://www.xyz.com/dashboard
login as admin and in left menu under settings you will find permissions)
Let us know if that works for you. I am always available on skype and whatsapp if you need any further support.