Disable direct links and proxy the content, that way you can create unique URIs for every buyer. Then you can do all sorts of things like add a watermark on the

Disable direct links and proxy the content, that way you can create unique URIs for every buyer. Then you can do all sorts of things like add a watermark on the content page containing the credit card number of the buyer or just add expiration time to the links. That'll limit sharing somewhat, although I don't think it's a huge problem.