Do I understand correctly that using Document Seller, you can have the documents stored on another server and then use an absolute path url to the location? If so, are both methods of sale deliver available: to download the file with a link emailed and/or to attach the file to an email?
Your site should be on the same server so that It could be protected. If your files are on a remove server, people can share the link to that file and everyone can download it