Mercora sounds cool - the best so far - but it still doesn't give any money to the users, so where's the carrot?

Mercora, a derivation of a Latin word meaning "to trade," is designing a site that will allow users to voluntarily open up their hard drives to millions of users. If members give permission, other users can find out what they just bought and what they're listening to at a given moment. Members can send each other clips from songs the labels want to promote, or even entire albums from up-and-coming bands that want their names known.