Originally Posted by Sly

cPanel and other similar panels allow for easy importing and exporting, making site migrations easier.

The guy you are purchasing the site from should not need access to your side, but you should need access to his side. Have your web hosting company do the migration. You will need his cPanel account access or access to the specific user that is controlling the domain/website you are buying.

Another option would be the seller can create an export of the site and then send it to you, which you would then import into your server.