I don't understand the advantage of using cloud storage instead of uploading a file. Unless I were selling a 4GB file and it was already in Dropbox and I was in a hurry...
Dropbox and Google Drive made uploading file super easy. You just drag&drop file on your hard drive and they take care about the rest. In addition to that, whenever you are going to update the file, you don't have to login. Again just drag&drop and Dropbox + Sellbox do the magic.
I agree it's convenient, but I'm having a hard time imagining people who would be selling digital content, if only they didn't have to upload it to a storefront...
Are there any generic examples of a digital storefront that you can easily self-publish files (preferably up to 25MB or so)? I'm not currently aware of any (although I haven't actually tried to search them out).