It's fine by me. Some people have different attitudes about children and pets... to some people, the pets are part of the family, to others, the pet is there to serve them.
Maybe the dog isn't particularly gentle with your child or you find yourself nervous about the dog being around the kid. You're not attached at all, really. If that's how you feel, why keep the dog?
I don't care that he sold it on twitter.
I take the "must be gone by tomorrow" and "bid on him" comments with a grain of salt. If he really was going to send the dog to the animal shelter or euthanize it the next day, of course I'd feel differently, but he was probably just mimicking a common technique for generating interest without thinking much about it.
Not ideal, but not the end of the world as long as he used some common sense when meeting the buyer. It would be worse if it was from a "dog person," but for an average pet owner that probably bought the dog under similar circumstances... eh, I have a hard time getting worked up about it.