Don’t sweat immediate legal action. The first step is typically a “cease and desist” threat from an attorney telling you to remove the copyrighted material or face legal consequences. The thing you want to worry about is building an internet business on a flawed foundation. Don’t get caught up in the romance of building the next Youtube only to find out everything your site contains is subject for removal. Read up on copyright law before you buy domains you hope to base a sharing service upon. It could mean having to adjust your Internet aspirations, but then again anything is better than getting sued by someone who can afford an armada of attorneys.