Steem is the whole system that is build on a blockchain. It's open source and anybody can use it anyway they want.

Steemit is the company that has designed and launched Steem. Steemit also runs this website (steemit.com) that is the first user interface for the Steem blockchain. In the future there will be other UIs too (stand-alone software, mobile apps, other websites) made by other people.

So yes, Steemit can be sold off. But that doesn't necessarily has any effect on Steem. The blockchain will continue to exists and function, it's not in any fundamental way depending on Steemit. Of course most of the developers work for Steemit, so in a practical way Steem is somewhat dependent on Steemit, at least for now.