I'm all for making well designed, useful products. But refusing to (at least) sell the product to another company because you weren't sure they "would do do right by our customers and the industry" just seems dumb to me.
Also, they have a product called Postmark in the same space, Why create a competitor for yourself?
It's their decision to sell or not. They did right by their existing customers in first making an honest go of the business, then cutting the price when that didn't work, and then getting a transition plan for closing it when the lower price brought too many low quality (ie: borderline spammer) customers.
You can have an opinion if that's right or not, but ultimately its their decision.
I understand that it's out of their hands once they sell it but if somebody made a good offer to buy the piece of tech then I (emphasis on the "I") would accept. The actions of the buyer doesn't dent their reputation IMHO as its no longer their product.