So if 37 Signals is more popular than you, what value there in paying $480,000 for a site whose customers may not be interested in it after 37 Signals leaves?

The brand value because of the 37 signals past cachet. The same reason brand x can be owned by random hedge fund company and people will still buy it.

Exactly, so you should expect some customers will leave once there is a sale, perhaps a significant amount. And if there is no sale they will be closing shop on July 1, so 37signals won't compete with your product if you build an alternative.