Extensions supporting the Dropbox API, allowing you to manage your Dropbox files through your Joomla admin, and make them available for sale through Virtuemart
Dropbox Extensions
AS3 Cloud File Manager
Manage cloud files stored on Amazon S3 or Dropbox from your Joomla Admin. Available as a free download (download button is lower down the page) with paid support
Spiral Paid Download Plugin for Virtuemart
Paid download plugin for Virtuemart 2 and 3. Sell your files, either locally hosted or cloud hosted on Amazon S3 or Dropbox