/sellacious redirects you to sellacious backend view which is for admin activities like adding products, tax and shipping rules etc. This is useful for admin, seller, staff etc but not customers thats why we do not allow customer login in sellacious backend.
But if you really want to allow customers backend login it can be done through permissions. For this you have to enable Sellacious backend login for clients (client category) and same login form can be used for customer login.
Again it is not advisable to allow the customers sellacious backend login.
Hope this helps,