If I was a buyer with a slow internet connection and I want to download a big file, that might cause some problem. A day or a week is probably better for the minimum limit.
A simple workaround (for the seller) would be to upload the files to Google drive or Dropbox for instance, and then lock that download link.
@OP more suggestions:
- I tried to upload a large file and the page (the browser) kept loading with no messages being displayed. You should add some loading animation to show the progress. Also, you should mention the maximum size of the upload, I'm sure you have one.
- I went to the "account" page to update my email alerts (uncheck them) and whenever I try to do that, I get an error "empty new password". You should not assume that the user is trying to change his password just because he's on the settings page.