OK, to be 100% clear on this, my suggestion is that you include metadata about the purchaser int he tags, so that (short of them scrubbing it, which is of course possible, but essentially akin to someone circumventing your DRM, which is also possible) you are able to determine who uploaded unauthorized copies to filesharing sites. This is what most DRM-free sellers do.
I'm not going to bother talking in circles about this because I've said everything I wanted to say and you're just making me repeat myself by not reading what I'm saying very closely.
We already talked about your suggestion internally to add the metadata with the user information into the movie so we know who shared the movie.
The problem is the only data we ask is an email and password. Anyone is able to create a new email account. The name in the credit card information can be also avoid with prepaid/gift cards so we do not have really any data to add to the movie to know the user who uploaded the movie to a file sharing service.