That login is your same joomla login. this helps you to manage your shoppingcart without logging inside your joomla admin panel, you can also provide your employee separate login details to manage shopping cart, and sellers with different login details this way they can manage shopping cart without essentially peeping inside your main admin.
you can also change the folder name of your sellacious admin panel, example you want your shoppingcart admin to be yourwebsite.com/backoffice just rename the folder and login inside newly changed admin panel and it will automatically configure itself to new name.
Super admin details will work on that login page by default, As a starting point you can use your super admin details, sellacious admin has easy switch between joomla admin panels, in case you want to access that.
your post was eye opener for us and made us realize that your confusion is valid and thus in next upcoming version we are making it more informative.
Moreover now no registration will be required (from next version) licensing will be seamless and not essential. this means you can directly install Sellacious from your Joomla Backend Web Install feature.
Let us know if you have any other suggestions. we will try to include it upcoming versions.
This is public free and open project, and users like you are our strength.
Looking forward for any further help.