Your idea is good, your minded me of the old X-Drive service in the late 1990s.
However, there are flaws with your concept:
If you can upload digital copies of the movie, chances are they are illegal (publishers will find ways to make them illegal). Legal downloads usually have DRM on there, which make your service useless.
Discuz HK and Uwants users are posting file with "fly-by" file hosting such as Megaupload/RapidShare/Zshare, so everything is anoymous and hard to trace. I know people who pay monthly for unlimited access to these file hosting (so files do not expire and unlimited download), pretty much like I subscribe to Usenet because I download a lot.
If you have a centralized server for the files, it's not hard to trace it back to you. You will be 100% liable because you provided this service. Even by invitation, you won't have the man power to monitor each accounts/usage and who are the users.
Finally, bandwidth are expensive. In order for you to offer downloads of large files, you will need a dedicated servers with lots of bandwidth. Unless you can find ways to finace this operation, you will be running a loss within a short amount of time.
You have past experiences with building a web store and forum, you should know how hard it is.
Don't mean to offense, I don't find any competitive advantages on your concept.