Hi my name is Joe I have a website Ecommerce listing business.
I work on moving listings to company's that just recently moved to
A new magento website.
Welcome to the community forums.
This community forums is a very good platform to ask the questions and try to help others.
You can also find related information on StackExchange and you can start diving into the http://devdocs.magento.com/
You can get idea about already reported/existing issue from Github: https://github.com/magento/magento2/issues
Coming back to your question - it depends what existing website 's platform is ? - if it is built on magento , then magento have produce data migration tool , using that you can easily move one website data to another website as an bulk.
Hope it helps !!