If you want to protect your project from being resold by potential competitors, do not release it as open source.
I think this problem might solve itself, though. Slowly but surely, companies and power users have become very wary of VC funded companies making big promises and big open source releases, with the knowledge that there is rarely a plan for sustainability and that there is a good chance if they stand on that rug it could be pulled later. Soon, if trends continue, the advantages that you once got from announcing something as open source will start to evaporate and turn into a liability as people start seeing ahead to the eventual "but of course we have to be able to monetize this eventually" stage.
The way I see it, a project can always be open sourced later on once there's a way to do it and ensure the company can remain sustainable. For the flagship product of a company, especially a VC-funded company, not starting open source is the ethical thing to do.
