Steve Jobs was a serial entrepreneur—starting Apple, then Pixar, then NeXT, then returning to transform Apple. Maybe he didn't want to be at the time, but in his Stanford commencement speech he said (with the benefit of retrospect) that getting fired from Apple was one of the most important events in his life. Here's what he said:

"I didn't see it then, but it turned out that getting fired from Apple was the best thing that could have ever happened to me. The heaviness of being successful was replaced by the lightness of being a beginner again, less sure about everything. It freed me to enter one of the most creative periods of my life."

Now, Steve didn't flip Apple, he didn't sell out. But the point is that he did start over from scratch. 37signals are right that building a business for the principal purpose of flipping it is probably a perverse goal. But being a serial entrepreneur is not a bad thing, and Jobs is precisely an example of why it's not a bad thing.