I believe Steemit.com itself can be bought, but the underlying STEEM platform cannot, because it would be like someone "buying the Bitcoin platform". It doesn't work like that. It would be more like someone buying Bitcoin.com, or Coinbase.com

As the STEEM economy increases, there will likely be strong alternatives to Steemit.com. There already is one in the form of the Esteem app, which I don't think has anything to do with Steemit.com at all.