That's not to say Dropbox would do that. But it probably would be nice to explicitly say somewhere that you won't sell or give away the physical bits that are uploaded to the service.

pasted from terms:

Dropbox does not claim any ownership rights in Your Files. You acknowledge that Dropbox does not have any obligation to monitor the Files or User Posts that are uploaded, posted, submitted, linked to or otherwise transmitted using the Site or Services, for any purpose and, as a result, is not responsible for the accuracy, completeness, appropriateness, legality or applicability of the Files or anything said, depicted or written by users in their User Posts, including without limitation, any information obtained by using the Site or Services. Dropbox does not endorse anything contained in the Files or User Posts or any opinion, recommendation or advice expressed therein and you agree to waive, and hereby do waive, any legal or equitable rights or remedies you have or may have against Dropbox with respect thereto.

Do you need to claim ownership of something in order to "share" it with a third party? (strictly legally speaking, of course)

The rest of the paragraph is just covering of your back. This is fine, of course, just not relevant to the point.