Maybe you should re-phrase this to ask, "How could I sell competitors the data if they want to buy it, but restrict them if they do not?". To me the licenses found in the Creative Commons accommodate this. They give you the licensing tools to flexibly control content and can let users to share content. Safe that in the knowledge competitors do not have the copyright rights to use/abuse licensed content without recourse to the law (in a lot of cases with laws adopted in your home country).
You cannot however use CC for 'facts and ideas' (as per CC FAQ, How does a Creative Commons license operate?) ~ http://wiki.creativecommons.org/FAQ
Having said that I know of no known tools or tool sets to automatically check for content, it's licensing (if any) and contact details for buyers. Buyers would grab at this seeking for free content (images for instance) clear of any copyright issues.
If you really don't want competitors (are your users competitors as well?) to take away your content then don't create API's allowing users to use their own content. But then you risk making another "Roach Motel" allowing users to create but not export. Is this what you mean, lockout of users data?
Excellent. Do you intend to go as far as say flickr and use an open API as well at some stage? This allows you to integrate third party services but at the potential competitor raids. Stuart Butterfield has lots to say on this particular problem ~ http://flickr.com/groups/central/discuss/72157594165399644/#...
Out of curiosity can you define "data"? I notice smugmug undertakes the type of restrictions you are describing in their pro account.
As a start you can review the open source licenses commonly used: http://en.wikipedia.org/wiki/Open-source_license
Or in theory you could always write your own license.. But once you open your source it will be impossible to completely protect your IP. People will find ways to copy it anyway..